Filed Pursuant to Rule 424(b)(5)
Registration No. 333-200686

PROSPECTUS SUPPLEMENT

(To Prospectus dated December 19, 2014)

Up to $15,000,000

TANDEM DIABETES CARE, INC.

Common Stock

We have entered into an Equity Distribution Agreement with Piper Jaffray & Co., or Piper Jaffray, relating to our common stock offered by this prospectus supplement. In accordance with the terms of the Equity Distribution Agreement, we may offer and sell our common stock having an aggregate offering price of up to $15,000,000 from time to time through Piper Jaffray acting as our agent.

Our common stock is listed on the NASDAQ Global Market under the symbol “TNDM.” On July 25, 2017, the last reported sale price of our common stock as reported on the NASDAQ Global Market was $0.67 per share.

Sales of our common stock, if any, under this prospectus supplement may be made in sales deemed to be “at-the-market” equity offerings as defined in Rule 415 promulgated under the Securities Act of 1933, as amended, or the Securities Act, including sales made directly on or through the NASDAQ Global Market, the existing trading market for our common stock, sales made to or through a market maker other than on an exchange, in negotiated transactions at market prices prevailing at the time of sale or at prices related to such prevailing market prices, and/or any other method permitted by law. Piper Jaffray will act as sales agent and will use commercially reasonable efforts to sell on our behalf all of the shares requested to be sold by us, consistent with its normal trading and sales practices, on mutually agreed terms between Piper Jaffray and us.  There is no arrangement for funds to be received in any escrow, trust or similar arrangement.

Piper Jaffray will be entitled to compensation at a fixed commission rate of 3.0% of the gross sales price per share sold through it as agent under the Equity Distribution Agreement.

In connection with the sale of our common stock on our behalf, Piper Jaffray will be deemed to be an “underwriter” within the meaning of the Securities Act and the compensation of Piper Jaffray will be deemed to be underwriting commissions. We have also agreed to provide indemnification and contribution to Piper Jaffray with respect to certain liabilities, including liabilities under the Securities Act.

Investing in our common stock involves a high degree of risk. See “Risk Factors” beginning on page S-8 of this prospectus supplement and on page 28 of our Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2017.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of the securities being offered by this prospectus supplement or accompanying prospectus, or determined if this prospectus supplement or accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

____________________________

Piper Jaffray

The date of this prospectus supplement is July 27, 2017.

TABLE OF CONTENTS

	Page
PROSPECTUS SUPPLEMENT
ABOUT THIS PROSPECTUS SUPPLEMENT	S-2
PROSPECTUS SUPPLEMENT SUMMARY.	S-3
RISK FACTORS	S-8
CAUTIONARY NOTE REGARDING FORWARD-LOOKING INFORMATION	S-10
USE OF PROCEEDS	S-11
DESCRIPTION OF COMMON STOCK	S-12
PRICE RANGE OF COMMON STOCK	S-12
DIVIDEND POLICY	S-14
DILUTION	S-15
CERTAIN U.S. FEDERAL TAX CONSIDERATIONS APPLICABLE TO HOLDERS OF COMMON STOCK	S-17
PLAN OF DISTRIBUTION	S-22
LEGAL MATTERS	S-23
EXPERTS	S-23
INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE	S-24
WHERE YOU CAN FIND MORE INFORMATION	S-25

PROSPECTUS
ABOUT THIS PROSPECTUS	1
SPECIAL NOTE REGARDING FORWARD-LOOKING INFORMATION	2
ABOUT THE COMPANY.	3
RISK FACTORS	4
USE OF PROCEEDS	5
THE SECURITIES WE MAY OFFER	6
DESCRIPTION OF CAPITAL STOCK	7
DESCRIPTION OF WARRANTS	9
DESCRIPTION OF UNITS	10
PLAN OF DISTRIBUTION	11
LEGAL MATTERS	13
EXPERTS	13
INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE	14
WHERE YOU CAN FIND MORE INFORMATION	15

S-1

ABOUT THIS PROSPECTUS SUPPLEMENT

This document is part of a registration statement that was filed with the Securities and Exchange Commission, or the SEC, using a “shelf” registration process and consists of two parts. The first part is the prospectus supplement, including the documents incorporated by reference herein, which describes the specific terms of this offering. The second part, the accompanying prospectus, including the documents incorporated by reference therein, provides more general information. In general, when we refer only to the prospectus, we are referring to both parts of this document combined. Before you invest, you should carefully read this prospectus supplement, the accompanying prospectus, all information incorporated by reference herein and therein, as well as the additional information described under the heading “Where You Can Find More Information.” These documents contain information you should carefully consider when deciding whether to invest in our common stock.

This prospectus supplement may add, update or change information contained in the accompanying prospectus. To the extent there is a conflict between the information contained in this prospectus supplement and the accompanying prospectus, you should rely on information contained in this prospectus supplement, provided that if any statement in, or incorporated by reference into, one of these documents is inconsistent with a statement in another document having a later date, the statement in the document having the later date modifies or supersedes the earlier statement. Any statement so modified will be deemed to constitute a part of this prospectus only as so modified, and any statement so superseded will be deemed not to constitute a part of this prospectus.

You should rely only on the information contained in this prospectus supplement, the accompanying prospectus, any document incorporated by reference herein or therein, or any free writing prospectuses we may provide to you in connection with this offering. Neither we nor Piper Jaffray has authorized anyone to provide you with any different information. We take no responsibility for, and can provide no assurance as to the reliability of, any other information that others may provide to you. The information contained in this prospectus supplement, the accompanying prospectus, and in the documents incorporated by reference herein or therein is accurate only as of the date such information is presented. Our business, financial condition, results of operations and prospects may have changed since that date.

This prospectus supplement and the accompanying prospectus do not constitute an offer to sell or the solicitation of an offer to buy any securities other than the shares of common stock to which it relates, nor do this prospectus supplement and the accompanying prospectus constitute an offer to sell or the solicitation of an offer to buy securities in any jurisdiction to any person to whom it is unlawful to make such offer or solicitation in such jurisdiction.

Unless otherwise indicated, information contained in or incorporated by reference into this prospectus concerning our industry and the markets in which we operate, including market opportunity, market position and competitive landscape, is based on information from our management’s estimates, as well as from industry publications, surveys and studies conducted by third parties. Management estimates are derived from publicly available information, our knowledge of our industry, and assumptions based on such information and knowledge, which we believe to be reasonable. In addition, while we believe that information contained in the industry publications, surveys and studies has been obtained from reliable sources, the accuracy and completeness of such information is not guaranteed, and we have not independently verified any of the data contained in these third-party sources.

This prospectus supplement and the accompanying prospectus, and any documents incorporated by reference herein or therein, include statements that are based on various assumptions and estimates that are subject to numerous known and unknown risks and uncertainties. Some of these risks and uncertainties are described under the heading “Risk Factors” beginning on page S-8 of this prospectus supplement and in our Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2017, which is incorporated by reference into the prospectus. These and other important factors could cause our future results to be materially different from the results expected as a result of, or implied by, these assumptions and estimates. You should read the information contained in this prospectus supplement and the accompanying prospectus, and the documents incorporated by reference herein and therein, completely and with the understanding that future results may be materially different and worse from what we expect. See the information included under the heading “Cautionary Note Regarding Forward-Looking Information.”

Securities offered pursuant to the registration statement to which this prospectus supplement relates may only be offered and sold if not more than three years have elapsed since December 19, 2014, the initial effective date of the registration statement, subject to the extension of this period in compliance with applicable SEC rules.

S-2

PROSPECTUS SUPPLEMENT SUMMARY

This prospectus supplement summary discusses the key aspects of the offering and highlights certain information appearing elsewhere in this prospectus supplement and the accompanying prospectus, and in the documents we incorporate by reference herein and therein. However, as this is a summary, it does not contain all of the information that you should consider before deciding to invest in our common stock. You are encouraged to carefully read the entire prospectus, including the information provided (i) under the heading “Risk Factors” in this prospectus supplement and in our Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2017, or the Q2 Quarterly Report, as may be updated by other filings we make with the SEC after the date of this prospectus supplement, and (ii) under the heading “Management’s Discussion and Analysis of Financial Condition and Results of Operations”, and our financial statements and the related notes, in our Annual Report on Form 10-K for the fiscal year ended December 31, 2016, the Q2 Quarterly Report, and the other periodic report filings we make with the SEC after the date of this prospectus supplement.

Unless otherwise stated in this prospectus supplement and the accompanying prospectus, references to “Tandem,” “we,” “us,” or “our” refer to Tandem Diabetes Care, Inc.

Overview

We are a medical device company with an innovative approach to the design, development and commercialization of products for people with insulin-dependent diabetes. We believe that our competitive advantage is rooted in our unique consumer-focused approach and proprietary technology platform. This allows us to deliver innovative hardware and software solutions to meet the various needs and preferences of people with diabetes and their healthcare providers. We manufacture and sell insulin pump products in the United States that are designed to address large and differentiated segments of the insulin-dependent diabetes market. Our insulin pump products include:

• the t:slim X2 Insulin Delivery System, or t:slim X2, our next-generation flagship product,

• the t:flex Insulin Delivery System, or t:flex, for people with greater insulin needs, and

• the t:slim G4 Insulin Delivery System, or t:slim G4, the first continuous glucose monitoring, or CGM, enabled pump with touchscreen simplicity.

We have shipped more than 57,000 insulin pumps since our initial launch in August 2012 and more than 50,000 pumps in the four years ended June 30, 2017. For the past three consecutive years, our company has been ranked #1 by insulin pump users in the United States for customer support in an independent survey by dQ&A, a leading diabetes research firm.

We have successfully launched four insulin pump products, beginning with the commercialization of our first insulin pump product, the t:slim Insulin Delivery System, or t:slim, in August 2012. During 2015, we commenced commercial sales of two additional insulin pumps: t:flex in May 2015 and t:slim G4 in September 2015. In October 2016, we commenced commercial sales of t:slim X2, and discontinued new sales of t:slim. t:slim, t:slim X2 and t:flex are compatible with the Tandem Device Updater, a revolutionary new tool that allows pump users to update their pumps’ software quickly and easily from a personal computer. The Tandem Device Updater provides our customers access to new and enhanced features faster than the industry has been able to in the past. Its first cleared use by the U.S. Food and Drug Administration, or FDA, was to update t:slim Pumps purchased before April 2015 to the latest software. In the first quarter of 2017, we filed a regulatory submission with the FDA to permit t:slim X2 customers to update their pumps’ software using the Tandem Device Updater to allow integration with the DexCom G5 Mobile CGM system. Subject to FDA approval, we intend to offer this update to t:slim X2 customers free of charge. In the future, this tool has the potential to enable users to add other new features and functionality to their pumps, such as automated insulin delivery, or AID, algorithms, independent of the typical four-year insurance pump replacement cycle.  

Our innovative approach to product design and development is consumer-focused and based on our extensive market research, as we believe the user is the primary decision maker when purchasing an insulin pump. Our market research consists of interviews, focus groups and online surveys to understand what people with diabetes, their caregivers and healthcare providers are seeking in order to improve diabetes therapy management. We also apply the science of human factors to our design and development process, which seeks to optimize our devices, allowing users to successfully operate our devices in their intended environment.

S-3

We developed our products to provide the specific features that people with insulin-dependent diabetes seek in a next-generation insulin pump. Our proprietary pumping technology allows us to design the slimmest and smallest durable insulin pumps on the market, without sacrificing insulin capacity. Our insulin pump platform features our patented Micro-Delivery technology, and a miniaturized pumping mechanism that draws insulin from a flexible bag within the pump’s cartridge, rather than relying on a syringe and plunger mechanism. It also features an easy-to-navigate software architecture, a vivid color touchscreen and a micro-USB connection that supports a rechargeable battery, software updates through the Tandem Device Updater, and uploads to t:connect Diabetes Management Application, or t:connect. t:connect is our custom cloud-based data management application that provides customers and healthcare providers a fast, easy and visual way to display therapy management data from the pump and supported blood glucose meters. Our next generation pump, t:slim X2, also features an advanced Bluetooth radio capable of communicating with multiple compatible devices.

We have rapidly increased sales since our commercial launch by expanding our sales, clinical and marketing infrastructure, by developing, commercializing and marketing multiple differentiated products that utilize our technology platform and consumer-focused approach, and by providing strong customer support. We believe that by demonstrating our product benefits and the shortcomings of existing insulin therapies, more people will choose our insulin pumps for their therapy needs, allowing us to further penetrate and expand the market. We also believe we are well positioned to address consumers’ needs and preferences with our current products and products under development and by offering customers a pathway to our future innovations through the Tandem Device Updater as they are approved by the FDA.

Products under Development

Our products under development support our strategy of focusing on both consumer and clinical needs. We intend to leverage our consumer-focused approach and proprietary technology platform to continue to develop products that have the features and functionality that will allow us to target people in different segments of the insulin-dependent diabetes market. Our current pump products under development include:

• t:slim X2 with G5 integration – This product, which is currently under FDA review, is designed to display DexCom G5 CGM sensor information directly on the pump Home Screen. We intend to launch the product within thirty (30) days following approval.

• t:slim X2 with PLGS – Our first generation AID system is expected to include a predictive low glucose suspend, or PLGS, algorithm that utilizes DexCom G5 sensor values. During 2016, we completed a feasibility study of our PLGS algorithm. The data from this feasibility study was used in an IDE submission for a pivotal study, which was approved by the FDA in May 2017. We expect to begin a pivotal study in the third quarter of 2017 and anticipate that it will conclude by the end of 2017. Once reports from the clinical trial sites are finalized, we intend to use the results in a PMA submission with the FDA. Based on this timing, our goal is to launch the product in the summer of 2018.

• t:slim X2 with TypeZero – Our second generation AID system is expected to integrate the t:slim X2 pump with the treat-to-range technology that we licensed from TypeZero Technologies LLC, as well as DexCom’s G6 sensor. With TypeZero’s technology, our product is intended to both increase and decrease basal insulin based on a person’s predicted blood glucose levels, as well as deliver automated correction boluses. In November 2016, we announced that we are working with DexCom and TypeZero on the integration of our technologies into the National Institute of Health funded International Diabetes Closed Loop Trial, or IDCL Trial. We anticipate that a portion of the trial will utilize a t:slim X2 integrated with TypeZero’s inControl AID algorithms that is designed to automatically adjust a person’s insulin based on information from a DexCom G6 sensor. We intend to use the results from this trial in a PMA submission with the FDA. Subject to both the timely completion of a successful IDCL Trial and future FDA approval, our goal is to launch this product by the end of 2018.

• t:sport Insulin Delivery System – This product will be half the size of t:slim and is being designed for people who seek even greater discretion and flexibility with the use of their insulin pump. At this time, there is not a predicate device for an insulin pump wirelessly controlled through a mobile device application. The timing of the commercialization of this product will be based on our prioritization of resources and ongoing dialogue with the FDA.

Beginning in the third quarter of 2017, we intend to replace the standard Luer-lok connector that currently joins an infusion set to our cartridge with a custom connector, the t:lock™ Connector.

Recent Developments

S-4

Requirement to Raise Additional Capital

On the date our financial statements included in the Q2 Quarterly Report were issued, our management believed that we did not have sufficient cash to fund our operations for the next twelve months without additional financing, and therefore, we concluded there was substantial doubt about our ability to continue as a going concern within one year after the date those financial statements were issued. In addition, the audit report and opinion of our independent registered public accounting firm contained in our financial statements for the year ended December 31, 2016, includes an explanatory paragraph that describes conditions that raise substantial doubt about our ability to continue as a going concern. 

Under our Amended and Restated Term Loan Agreement with Capital Royalty Partners II L.P. and its affiliate funds, or Capital Royalty Partners, as amended, which we refer to as the Term Loan Agreement, we are required to complete one or more financings in which our aggregate gross proceeds from the sale of equity securities is at least $30.0 million, no later than January 15, 2018. In March 2017, we completed a public offering of our common stock with gross proceeds of $23,125,000. However, we are required to sell additional equity securities in order to satisfy the covenant under the Term Loan Agreement. We are undertaking this offering for the purpose of satisfying the covenant, as well as to generate net proceeds for working capital and other general corporate purposes. However, there can be no assurance that we will be successful in raising sufficient gross proceeds from this offering to meet our obligations under the covenant.   

Regardless of the outcome of this offering, we expect we will be required to raise additional capital in order to continue as a going concern, meet our minimum liquidity requirements, and execute on our business strategy. We may seek additional capital from public or private offerings of our capital stock, or we may elect to borrow additional amounts under new debt financing arrangements or from other sources. We expect our ability to raise additional financing may be negatively impacted by a number of factors, including our recent and projected financial results, recent changes in and volatility of our stock price, perceptions about the completion of the March 2017 public offering of our common stock, perceptions about this offering, concerns regarding our ability to maintain the continued listing of our common stock on the NASDAQ Global Market, our current level of indebtedness and debt service costs, our conclusion that there is substantial doubt about our ability to continue as a going concern, the competitive environment in our industry, and accounting complexities brought about by our Technology Upgrade Program (which is discussed in more detail in the Q2 Quarterly Report). There can be no assurance that we will be able to raise additional capital on acceptable terms or at all, whether in this offering or otherwise. If we are not able to secure additional financing or generate sufficient revenues from the sale of our products, we may be forced to significantly alter our business strategy, substantially curtail or modify our current operations, or cease operations altogether. 

Special Stockholder Meeting to Approve Reverse Stock Split

On July 27, 2017, we filed a definitive proxy statement with the SEC that relates to a special meeting of our stockholders to be held on September 7, 2017. At the special meeting, our stockholders will be asked to vote on a proposal to approve an amendment to our amended and restated certificate of incorporation to effect a reverse stock split of our issued and outstanding shares of common stock at a ratio of not less than 1-for-8 and not greater than 1-for-12, with the exact ratio to be set within that range by our board of directors before June 20, 2018, without further approval or authorization of our stockholders.  Our board of directors may alternatively elect to abandon the amendment and not effect the reverse stock split authorized by stockholders, in its sole discretion. Upon the effectiveness of the amendment to our certificate of incorporation effecting the reverse stock split, the issued and outstanding shares of our common stock will be reclassified and converted into a lesser number of shares such that one share of our common stock will be issued for a specified number of shares within the approved reverse stock split range.  

The reverse stock split would not change the number of authorized shares of our common stock as set forth in our amended and restated certificate of incorporation. Therefore, because the reverse stock split would result in the number of issued and outstanding shares of common stock decreasing, the number of authorized shares of common stock remaining available for issuance would increase proportionately to the ratio of the reverse stock split. The reverse stock split will affect all of our stockholders uniformly and will not affect any stockholder’s percentage ownership interest in our company, except to the extent that the reverse stock split results in any of our stockholders owning a fractional share.

Please refer to the definitive proxy statement for a description of the various reasons our board of directors approved the reverse stock split, as well as the various risks associated with the reverse stock split. There can be no assurance that our stockholders will approve the amendment to our amended and restated certificate of incorporation to effect the reverse stock split and, even if our stockholders approve the amendment, there can be no assurance that our board of directors will elect to effect the reverse stock split.

S-5

The share numbers and other amounts set forth in this prospectus, and in the documents incorporated by reference into this prospectus, do not take into account the potential future impact of the reverse stock split.

Corporate Information

We were incorporated in Colorado in January 2006 and reincorporated in Delaware in January 2008. Our principal executive offices are located at 11045 Roselle Street, San Diego, California 92121. The telephone number of our principal executive office is (858) 366-6900. Our website is www.tandemdiabetes.com. The information contained on or accessed through our website is not incorporated by reference into this prospectus, and you should not consider information contained on our website to be a part of this prospectus or in deciding whether to purchase our common stock. References in this prospectus to our website are to inactive textual references only.

Implications of Being an Emerging Growth Company

We qualify as an “emerging growth company” as defined in the Jumpstart our Business Startups Act of 2012, or the JOBS Act. An emerging growth company may take advantage of specified reduced reporting requirements and is relieved of certain other significant requirements that are otherwise generally applicable to public companies. As an emerging growth company:

• we are exempt from the requirement to obtain an attestation and report from our auditors on the assessment of our internal control over financial reporting pursuant to the Sarbanes-Oxley Act of 2002;

• we are permitted to provide less extensive disclosure about our executive compensation arrangements; and

• we are not required to give our stockholders non-binding advisory votes on executive compensation or golden parachute arrangements.

We may take advantage of these provisions until such time that we no longer qualify as an emerging growth company. We will cease to be an emerging growth company upon the earliest of: (i) December 31, 2018, (ii) the last day of the first fiscal year in which our annual gross revenues exceed $1.07 billion, (iii) December 31 of the fiscal year that we become a “large accelerated filer” as defined in Rule 12b-2 under the Securities Exchange Act of 1934, or the Exchange Act, which would occur if the market value of our common stock held by non-affiliates exceeds $700 million as of the last business day of our most recently completed second fiscal quarter, or (iv) the date on which we have issued more than $1.0 billion in non-convertible debt during the preceding three-year period.

We may choose to take advantage of some but not all of these reduced burdens. We have taken advantage of reduced reporting requirements in this prospectus. Accordingly, the information contained herein may be different from the information you receive from our competitors that are public companies, or other public companies in which you have made an investment.

S-6

THE OFFERING

Issuer:	Tandem Diabetes Care, Inc.
Common Stock offered by us:	A number of shares of common stock resulting in aggregate gross proceeds to us of up to $15.0 million.
Manner of offering:	“At-the-market” offering that may be made from time to time through Piper Jaffray. See “Plan of Distribution” on page S-22.
Use of proceeds:	We intend to use the net proceeds from this offering for working capital and other general corporate purposes. See “Use of Proceeds” on page S-11.
Risk factors:	Investing in our common stock involves a high degree of risk. See the information included under the heading “Risk Factors” beginning on page S-8 of this prospectus supplement and in the Q2 Quarterly Report, which is incorporated by reference in this prospectus, for a discussion of factors that you should carefully consider before deciding to invest in our common stock.
NASDAQ Global Market symbol:	“TNDM”

S-7

RISK FACTORS

Investing in our common stock involves a high degree of risk. Before making an investment decision, you should carefully consider the risks described below, together with all of the other information included in this prospectus supplement, the accompanying prospectus, and the information incorporated by reference herein and therein, including the risks described under the heading “Risk Factors” in the Q2 Quarterly Report, as may be updated by other filings we make with the SEC after the date of this prospectus supplement.

If any of the risks described below, or incorporated by reference into this prospectus, actually occur, our business, financial condition, results of operations and prospects could suffer. In that case, the trading price of our common stock may decline and you may lose all or part of your investment. The risks and uncertainties we have described are not the only ones we face. Additional risks and uncertainties not presently known to us or that we currently deem immaterial may also affect our business, financial condition results of operations and prospects. Certain statements below are forward-looking statements. See the information included under the heading “Cautionary Note Regarding Forward-Looking Information.”

Risks Related to this Offering

Purchasers in this offering will experience immediate and substantial dilution in the book value of their investment.

The offering price per share in this offering may exceed the pro forma net tangible book value per share of our common stock outstanding prior to this offering. Assuming that an aggregate of 22,388,060 shares of our common stock are sold at an average offering price of $0.67 per share, the last reported sale price of our common stock on the NASDAQ Global Market on July 25, 2017, for aggregate gross proceeds of $15.0 million, and after deducting commissions and estimated aggregate offering expenses payable by us, you will experience immediate dilution of $0.75 per share, representing the difference between our as adjusted net tangible book value (deficit) per share as of June 30, 2017 after giving effect to this offering and the assumed average offering price. The exercise of outstanding stock options or warrants could result in further dilution of your investment.

You may experience future dilution as a result of future equity offerings.

We expect we will be required to raise additional capital in order to continue as a going concern, meet our minimum liquidity requirements, and execute on our business strategy. In order to raise additional capital, we may in the future offer additional shares of our common stock or other securities convertible into or exchangeable for our common stock at prices that may not be the same as the price per share paid by investors in this offering. We may sell shares or other securities in any other offering at a price per share that is less than the price per share paid by any investors in this offering, and investors purchasing shares or other securities in the future could have rights superior to existing stockholders. The price per share at which we sell additional shares of our common stock, or securities convertible or exchangeable into common stock, in future transactions may be higher or lower than the price per share paid by any investors in this offering.

It is not possible to predict the actual number of shares we will sell under the Equity Distribution Agreement, or the gross proceeds resulting from those sales.

Subject to certain limitations in the Equity Distribution Agreement and compliance with applicable law, we have the discretion to deliver a placement notice to Piper Jaffray at any time throughout the term of the Equity Distribution Agreement. The number of shares that are sold through Piper Jaffray after delivering a placement notice will fluctuate based on a number of factors, including the market price of the common stock during the sales period, the limits we set with Piper Jaffray in any applicable placement notice, and the demand for our common stock during the sales period.  Because the price per share of each share sold will fluctuate during the sales period, it is not currently possible to predict the number of shares that will be sold or the gross proceeds to be raised in connection with those sales.

S-8

We may not be successful in raising sufficient gross proceeds from the sale of shares in this offering to satisfy the covenant in our Term Loan Agreement.

Under our Term Loan Agreement, we are required to complete one or more financings in which our aggregate gross proceeds from the sale of equity securities is at least $30.0 million, no later than January 15, 2018. In March 2017, we completed a public offering of our common stock with gross proceeds of $23,125,000. However, we are required to sell additional equity securities sufficient to raise a minimum of an additional $6,875,000 of gross proceeds in order to satisfy the covenant under the Term Loan Agreement.  While we are undertaking this offering for the purpose of satisfying the covenant, there can be no assurance that we will be successful in raising sufficient gross proceeds from this offering, or from any future offering, to meet our obligations under the covenant.   

The common stock offered hereby will be sold in “at-the-market” offerings, and investors who buy shares at different times will likely pay different prices.

Investors who purchase shares in this offering at different times will likely pay different prices, and so may experience different levels of dilution and different outcomes in their investment results. We will have discretion, subject to market demand, to vary the timing, prices, and numbers of shares sold in this offering. In addition, subject to the final determination by our board of directors, there is no minimum or maximum sales price for shares to be sold in this offering. Investors may experience a decline in the value of the shares they purchase in this offering as a result of sales made at prices lower than the prices they paid.

We have broad discretion to determine how to use the funds raised in this offering, and may use them in ways that may not enhance our operating results or the price of our common stock.

Our management will have broad discretion over the use of proceeds from this offering, and we could spend the proceeds from this offering in ways our stockholders may not agree with or that do not yield a favorable return. We intend to use the net proceeds of this offering for working capital and other general corporate purposes. However, our use of these proceeds may differ substantially from our current plans. If we do not invest or apply the proceeds of this offering in ways that improve our operating results, we may fail to achieve expected financial results, which could cause our stock price to decline.

S-9

CAUTIONARY NOTE REGARDING FORWARD-LOOKING INFORMATION

This prospectus contains “forward-looking statements” within the meaning of the federal securities laws, which statements are subject to considerable risks and uncertainties. These forward-looking statements are intended to qualify for the safe harbor from liability established by the Private Securities Litigation Reform Act of 1995. All statements included or incorporated by reference in this prospectus, other than statements of historical fact, are forward-looking statements. You can identify forward-looking statements by the use of words such as “may,” “will,” “could,” “anticipate,” “expect,” “intend,” “believe,” “continue” or the negative of such terms, or other comparable terminology. Forward-looking statements also include the assumptions underlying or relating to such statements. In particular, forward-looking statements contained in this prospectus relate to, among other things, our future or assumed financial condition (including our ability to continue as a going concern to raise additional capital and to succeed in our future operations), results of operations, liquidity, business forecasts and plans, research and product development plans, manufacturing plans, strategic plans and objectives, capital needs and financing plans, product launches, regulatory approvals, competitive environment, and the application of accounting guidance. We caution you that the foregoing list may not include all of the forward-looking statements made in this prospectus.

Our forward-looking statements are based on our management’s current assumptions and expectations about future events and trends, which affect or may affect our business, strategy, operations or financial performance. Although we believe that these forward-looking statements are based upon reasonable assumptions, they are subject to numerous known and unknown risks and uncertainties and are made in light of information currently available to us. Our actual financial condition and results could differ materially from those anticipated in these forward-looking statements as a result of various factors, including those set forth below in the section entitled “Risk Factors” beginning on page S-8 of this prospectus supplement and in the Q2 Quarterly Report, which is incorporated by reference in this prospectus, as well as in the other reports we file with the SEC. You should read this prospectus with the understanding that our actual future results may be materially different from and worse than what we expect.

Moreover, we operate in an evolving environment. New risk factors and uncertainties emerge from time to time and it is not possible for our management to predict all risk factors and uncertainties, nor can we assess the impact of all factors on our business or the extent to which any factor, or combination of factors, may cause actual results to differ materially from those contained in any forward-looking statements.

Forward-looking statements speak only as of the date they were made, and, except to the extent required by law or the NASDAQ Listing Rules, we undertake no obligation to update or review any forward-looking statement because of new information, future events or other factors.

We qualify all of our forward-looking statements by these cautionary statements.

S-10

USE OF PROCEEDS

We may issue and sell our common stock having aggregate gross proceeds of up to $15.0 million from time to time. Because there is no minimum offering amount required as a condition to close this offering, the actual total public offering amount, commissions and proceeds to us, if any, are not determinable at this time. We estimate that the net proceeds from the sale of our common stock that we are offering may be up to $14.3 million, after deducting Piper Jaffray’s commission and estimated offering expenses payable by us.

We currently anticipate that we will use the net proceeds received by us for working capital and other general corporate purposes. Our expected use of the net proceeds from this offering is based upon our present plans and business condition. As of the date of this prospectus supplement, we cannot predict with certainty all of the particular uses for the net proceeds to be received upon the completion of this offering or the amounts that we will actually spend on the uses set forth above. The amounts and timing of our actual use of proceeds will vary depending on numerous factors, including the factors described under the heading “Risk Factors” beginning on page S-8 of this prospectus supplement and in the Q2 Quarterly Report, which is incorporated by reference in this prospectus. As a result, management will retain broad discretion over the allocation of the net proceeds from this offering, and investors will be relying on the judgment of our management regarding the application of the net proceeds.

Pending the use of the net proceeds, we intend to invest the net proceeds in high quality, short-term interest-bearing obligations, investment-grade instruments, certificates of deposit or direct or guaranteed obligations of the U.S. government.

S-11

DESCRIPTION OF COMMON STOCK

General

Our authorized capital stock consists of 100,000,000 shares of our common stock, $0.001 par value per share, and 5,000,000 shares of undesignated preferred stock, $0.001 par value per share. As of July 25, 2017, there were 50,228,931 shares of our common stock outstanding, which was held of record by approximately 71 stockholders, and there were no shares of our preferred stock outstanding.

Common Stock

Our common stock has been publicly traded on the NASDAQ Global Market under the symbol “TNDM” since our initial public offering in November 2013. On July 25, 2017, the last reported sale price of our common stock on the NASDAQ Global Market was $0.67.

On June 14, 2017, we received notice that we had failed to meet the NASDAQ minimum closing bid price requirement of $1.00 per share for 30 consecutive business days, which could subject our common stock to delisting. For more information, see "Risk Factors – Risks Related to our Common Stock – If we are unable to comply with certain continued listing requirements of NASDAQ, our common stock would be delisted from NASDAQ.*" in the Q2 Quarterly Report, which is incorporated herein by reference.

The material terms of our common stock are described under the heading “Description of Capital Stock” in the accompanying prospectus.

Price Range of Common Stock

The following table sets forth, for the periods indicated, the high and low intraday sale prices of our common stock as reported by the NASDAQ Global Market.

	Price Range
	High		Low
Year Ended December 31, 2017:
First Quarter	$	3.00	$	1.10
Second Quarter	$	1.30	$	0.76
Year Ended December 31, 2016:
First Quarter	$	11.80	$	6.59
Second Quarter	$	11.30	$	6.48
Third Quarter	$	8.81	$	6.04
Fourth Quarter	$	8.10	$	1.60
Year Ended December 31, 2015:
First Quarter	$	14.27	$	11.50
Second Quarter	$	14.19	$	10.54
Third Quarter	$	13.48	$	8.52
Fourth Quarter	$	12.48	$	7.26

Holders

As of July 25, 2017, there were approximately 71 holders of record of our common stock. The actual number of common stockholders is greater than the number of record holders, and includes stockholders who are beneficial owners, but whose shares are held in street name by brokers and other nominees. This number of holders of record also does not include stockholders whose shares may be held in trust by other entities.

S-12

Special Stockholder Meeting to Approve Reverse Stock Split

On July 27, 2017, we filed a definitive proxy statement with the SEC that relates to a special meeting of our stockholders to be held on September 7, 2017. At the special meeting, our stockholders will be asked to vote on a proposal to approve an amendment to our amended and restated certificate of incorporation to effect a reverse stock split of our issued and outstanding shares of common stock at a ratio of not less than 1-for-8 and not greater than 1-for-12, with the exact ratio to be set within that range by our board of directors before June 20, 2018, without further approval or authorization of our stockholders.

The share numbers and other amounts set forth in this prospectus, and in the documents incorporated by reference into this prospectus, do not take into account the potential future impact of the reverse stock split.

Additional information about the proposal to approve an amendment to our amended and restated certificate of incorporation to effect the reverse stock split is described under the heading “Prospectus Supplement Summary – Recent Developments”.

S-13

DIVIDEND POLICY

We have never declared or paid any cash dividends on our common stock. We have no current plans to declare or pay any dividends and intend to retain all of our future earnings, if any, generated by our operations for the development and growth of our business. Any future decision to pay dividends will be made by our board of directors in its sole discretion and will depend upon our results of operations, financial condition, capital requirements and other factors that our board of directors deems relevant in its informed business judgment. In addition, the terms of the Term Loan Agreement restrict our ability to pay cash dividends.

S-14

DILUTION

If you invest in our common stock in this offering, your ownership interest will be immediately diluted to the extent of the difference between the public offering price per share and the as adjusted net tangible book value per share after this offering. As of June 30, 2017, our net tangible book value (deficit) was $(19.9) million, or $(0.40) per share. Net tangible book value (deficit) per share represents our total tangible assets (excluding deferred issuance costs) less our total liabilities, divided by the number of shares outstanding.

After giving effect to the sale of 22,388,060 shares of our common stock in an amount equal to $15.0 million at an assumed average price of $0.67 per share, the last reported sale price of our common stock on the NASDAQ Global Market on July 25, 2017, and after deducting commissions to Piper Jaffray and estimated offering expenses payable by us, our as adjusted net tangible book value (deficit) as of June 30, 2017 would have been $(5.6) million, or $(0.08) per share.

This amount represents an immediate increase in net tangible book value (deficit) of $0.32 per share to our existing stockholders and immediate dilution in net tangible book value (deficit) of $0.75 per share to new investors purchasing shares in this offering. We have determined the amount of dilution by subtracting the as adjusted net tangible book value (deficit) per share after this offering from the assumed average price per share paid by an investor in this offering.

The following table illustrates this dilution.

Assumed average public offering price per share			$	0.67
Net tangible book value (deficit) per share as of June 30, 2017	$	(0.40)
Increase in net tangible book value (deficit) per share attributable to this offering		0.32
As adjusted net tangible book value (deficit) per share after this offering				(0.08)
Dilution per share to new investors in this offering			$	0.75

The following table summarizes, on the pro forma basis described above as of June 30, 2017, the differences between the number of shares of common stock purchased from us, the total consideration paid and the average price per share paid by existing stockholders and by investors purchasing in this offering at an assumed average price of $0.67 per share, the last reported sale price of our common stock on the NASDAQ Global Market on July 25, 2017, before deducting the commissions and estimated offering expenses payable by us.

	Shares Purchased					Total Consideration					Average Price Per Share
	Number		Percent			Amount		Percent
Existing stockholders before this offering		50,228,931		69.2	%	$	377,085,576		96.2	%	$	7.51
Investors purchasing in this offering		22,388,060		30.8			15,000,000		3.8		$	0.67
Total		72,616,991		100.0	%	$	392,085,576		100.0	%	$	5.40

The tables above assume, for illustrative purposes, that an aggregate of 22,388,060 shares of common stock are sold during the term of the Equity Distribution Agreement with Piper Jaffray at an average price of $0.67 per share, the last reported sale price of our common stock on the NASDAQ Global Market on July 25, 2017, for aggregate gross proceeds of $15.0 million. The shares subject to the Equity Distribution Agreement with Piper Jaffray are being sold from time to time at various prices. An increase of $0.25 per share in the average price at which the shares are sold from the assumed average offering price of $0.67 per share shown in the table above, assuming all of our common stock in the aggregate amount of $15.0 million during the term of the Equity Distribution Agreement with Piper Jaffray is sold at that price, would increase our adjusted net tangible book value (deficit) per share after the offering to $0.00 per share and would increase the dilution in net tangible book value (deficit) per share to new investors in this offering to $0.92 per share, after deducting commissions and estimated offering expenses payable by us. A decrease of $0.25 per share in the average price at which the shares are sold from the assumed average offering price of $0.67 per share shown in the table above,

S-15

assuming all of our common stock in the amount of $15.0 million during the term of the sales agreement with Piper Jaffray are sold at that price, would decrease our adjusted net tangible book value (deficit) per share after the offering to $(0.15)per share and would decrease the dilution in net tangible book value (deficit) per share to new investors in this offering to $0.57per share, after deducting commissions and estimated aggregate offering expenses payable by us. This information is supplied for illustrative purposes only and may differ based on the actual average offering price and the actual number of shares sold.

The number of shares of common stock outstanding in the table excludes:

• 2,927,921 shares of common stock issuable upon exercise of outstanding warrants as of June 30, 2017, at a weighted average exercise price of $4.05 per share;

• 1,524,958 shares of common stock issuable upon exercise of outstanding options to purchase shares of common stock under the 2006 Stock Incentive Plan as of June 30, 2017, at a weighted average exercise price of $2.44 per share (of which options to acquire 1,516,331 shares of common stock are vested as of June 30, 2017);

• 7,812,288 shares of common stock issuable upon exercise of outstanding options to purchase shares of common stock under the 2013 Stock Incentive Plan, or the 2013 Plan, as of June 30, 2017, at a weighted average exercise price of $7.71 per share (of which options to acquire 2,896,051 shares of common stock are vested as of June 30, 2017) and 396,904 shares that are reserved for future issuance under the 2013 Plan as of June 30, 2017; and

• 133 shares of common stock reserved for future grant or issuance under the ESPP as of June 30, 2017.

Our option holders and warrant holders may exercise the above-referenced options and warrants in the future or we may make future grants under the above-referenced plans. To the extent that any of these options or warrants are exercised, new options or shares of common stock are issued under the 2013 Plan or the ESPP, or we issue additional shares of common stock or other equity securities in the future, there will be further dilution to investors purchasing in this offering.

S-16

CERTAIN U.S. FEDERAL TAX CONSIDERATIONS APPLICABLE TO HOLDERS OF COMMON STOCK

The following is a description of certain U.S. federal income and estate tax considerations related to the purchase, ownership and disposition of our common stock that are applicable to U.S. and non-U.S. holders (defined below).

This summary:

• is based on the U.S. Internal Revenue Code of 1986, as amended, or the Code, U.S. federal tax regulations promulgated or proposed under it, or Treasury Regulations, judicial authority and published rulings and administrative pronouncements of the U.S. Internal Revenue Service, or IRS, each as of the date of this prospectus and each of which are subject to change at any time, possibly with retroactive effect;

• is applicable only to holders who hold the shares as “capital assets” within the meaning of section 1221 of the Code;

• does not discuss the applicability of any U.S. state or local taxes, non-U.S. taxes or any other U.S. federal tax except for U.S. federal income tax; and

• does not address all aspects of U.S. federal income taxation that may be relevant to holders in light of their particular circumstances including alternative minimum tax considerations or who are subject to special treatment under U.S. federal income tax laws, including but not limited to:

• certain former citizens and long-term residents of the United States;

• banks, financial institutions, or “financial services entities”;

• insurance companies;

• tax-exempt organizations;

• tax-qualified retirement and pension plans;

• brokers, dealers or traders in securities, commodities or currencies;

• persons subject to the alternative minimum tax;

• persons that own or have owned more than 5% of our common stock;

• persons who hold or receive our common stock pursuant to the exercise of any employee stock option or otherwise as compensation;

• investors holding our common stock as part of a “straddle,” “hedge,” “conversion transaction,” or other risk-reduction transaction;

• investors who are an integral parts or controlled entities of a foreign sovereign, partnerships or other pass-through entities;

• real estate investment trusts and regulated investment companies; and

• “controlled foreign corporations” and “passive foreign investment companies.”

This description constitutes neither tax nor legal advice. Prospective investors are urged to consult their own tax advisors to determine the specific tax consequences and risks to them of purchasing, holding and disposing of our common stock, including the application to their particular situations of any U.S. federal, state, local and non-U.S. tax laws and of any applicable income tax treaty.

S-17

Certain U.S. Federal Income Tax Considerations Applicable to U.S. Holders

U.S. Holder Defined

For purposes of this discussion, a U.S. holder is a beneficial owner of our common stock that is a “U.S. person” for U.S. federal income tax purposes. A “U.S. person” is any of the following:

• a citizen or resident of the United States for U.S. federal income tax purposes;

• a corporation, or other entity taxable as a corporation for U.S. federal income tax purposes, that was created or organized in or under the laws of the United States or any state thereof or the District of Columbia;

• an estate the income of which is subject to U.S. federal income tax regardless of its source; or  

• a trust if (a) a court within the United States is able to exercise primary supervision over its administration and one or more U.S. persons have the authority to control all substantial decisions of the trust, or (b) the trust has a valid election in effect to be treated as a U.S. person.

If a partnership (or an entity or arrangement treated as a partnership for U.S. federal income tax purposes) owns our common stock, then the U.S. federal income tax treatment of a partner in that partnership, including a partner that is a U.S. person, generally will depend on the status of the partner and the partnership’s activities. Partners and partnerships should consult their own tax advisors with regard to the U.S. federal income tax treatment of an investment in our common stock.

Distributions to U.S. Holders

Distributions of cash or property, if any, paid to a U.S. holder of our common stock will constitute dividends for U.S. federal income tax purposes to the extent paid from our current or accumulated earnings and profits, as determined under U.S. federal income tax principles. Distributions made on our common stock that are treated as dividends generally will be included in your income as ordinary dividend income. With respect to noncorporate taxpayers, such dividends are generally taxed at reduced rates provided certain holding period requirements are satisfied.

Amounts not treated as dividends for U.S. federal income tax purposes will constitute a return of capital and first be applied against and reduce a holder’s adjusted tax basis in its common stock, but not below zero. Any excess will be treated as capital gain and will be treated as described under the section titled “—Sale or Taxable Disposition of Common Stock by U.S. Holders” below.

Sale or Taxable Disposition of Common Stock by U.S. Holders

Upon the sale, exchange or disposition of our common stock, you generally will recognize capital gain or loss equal to the difference between (i) the amount of cash and the fair market value of any property received upon the sale or exchange and (ii) your adjusted tax basis in the common stock. Such capital gain or loss will be long-term capital gain or loss if your holding period in the common stock is more than one year at the time of the sale, exchange or disposition. Long-term capital gains recognized by certain noncorporate taxpayers will generally be subject to reduced rates of U.S. federal income tax. The deductibility of capital losses is subject to limitations.

Medicare Contributions Tax

Certain U.S. holders who are individuals, estates or certain trusts must pay a 3.8% tax on the U.S. person’s “net investment income.” Net investment income generally includes, among other things, dividend income and net gains from the disposition of our common stock. A U.S. holder that is an individual, estate or trust should consult its tax advisor regarding the applicability of the Medicare tax to its income and gains in respect of its investment in our common stock.

Certain U.S. Federal Income Tax Considerations Applicable to Non-U.S. Holders

Non-U.S. Holder Defined

For purposes of this discussion, a non-U.S. holder is a beneficial owner of our common stock that is not a “U.S. holder” (as defined under the section titled “U.S. Holder Defined” above).  

S-18

If a partnership (or an entity or arrangement treated as a partnership for U.S. federal income tax purposes) owns our common stock, then the U.S. federal income tax treatment of a partner, including a partner that is a non-U.S. person, in that partnership generally will depend on the status of the partner and the partnership’s activities. Partners and partnerships should consult their own tax advisors with regard to the U.S. federal income tax treatment of an investment in our common stock.

Distributions to Non-U.S. Holders

Distributions of cash or property, if any, paid to a non-U.S. holder of our common stock will constitute “dividends” for U.S. federal income tax purposes to the extent paid out of our current or accumulated earnings and profits, as determined for U.S. federal income tax purposes. If the amount of a distribution exceeds both our current and accumulated earnings and profits, such excess will first constitute a nontaxable return of capital, which will reduce the holder’s tax basis in our common stock, but not below zero. Any excess will be treated as gain from the sale of our common stock and will be treated as described under the section titled “—Sale or Taxable Disposition of Common Stock by Non-U.S. Holders” below.

Subject to the following paragraphs, dividends on our common stock generally will be subject to U.S. federal withholding tax at a 30% gross rate, subject to any exemption or lower rate as may be specified by an applicable income tax treaty. We may withhold up to 30% of either (i) the gross amount of the entire distribution, even if the amount of the distribution is greater than the amount constituting a dividend, as described above, or (ii) the amount of the distribution we project will be a dividend, based upon a reasonable estimate of both our current and our accumulated earnings and profits for the taxable year in which the distribution is made. If tax is withheld on the amount of a distribution in excess of the amount constituting a dividend, then you may obtain a refund of that excess amount by timely filing a claim for refund with the IRS. Any such distributions will also be subject to the discussion below under the section titled “Foreign Account Tax Compliance Act Considerations.”

To claim the benefit of a reduced rate of or an exemption from U.S. federal withholding tax under an applicable income tax treaty, a non-U.S. holder will be required (i) to satisfy certain certification requirements, which may be made by providing us or our agent with a properly executed and completed IRS Form W-8BEN (for individuals) or W-8BEN-E (for entities) certifying, under penalty of perjury, that the holder qualifies for treaty benefits and is not a U.S. person or (ii) if our common stock is held through certain non-U.S. intermediaries, to satisfy the relevant certification requirements of the applicable Treasury Regulations. Special certification and other requirements apply to certain non-U.S. holders that are pass-through entities. Non-U.S. holders that do not timely provide us or our paying agent with the required certification, but that qualify for a reduced treaty rate, may obtain a refund of any excess amounts withheld by timely filing an appropriate claim for refund with the IRS. Non-U.S. holders should consult their tax advisors regarding their entitlement to benefits under an applicable income tax treaty.

Dividends that are effectively connected with the conduct of a trade or business by the non-U.S. holder within the United States (and, if required by an applicable income tax treaty, are attributable to a permanent establishment, or a fixed base in the case of an individual non-U.S. holder, that is maintained by the non-U.S. holder in the United States) (“effectively connected dividends”) are not subject to the U.S. federal withholding tax, provided that the non-U.S. holder certifies, under penalty of perjury, that the dividends paid to such holder are effectively connected dividends on a properly executed and completed IRS Form W-8ECI (or other applicable  form). Instead, any such dividends will be subject to U.S. federal income tax on a net income basis in a manner similar to that which would apply if the non-U.S. holder were a U.S. person.

Corporate non-U.S. holders who receive effectively connected dividends may also be subject to an additional “branch profits tax” at a gross rate of 30% on their earnings and profits for the taxable year that are effectively connected with the holder’s conduct of a trade or business within the United States, subject to any exemption or reduction provided by an applicable income tax treaty.

Sale or Taxable Disposition of Common Stock by Non-U.S. Holders

Any gain realized on the sale, exchange or other taxable disposition of our common stock generally will not be subject to U.S. federal income tax unless:

• the gain is effectively connected with the conduct of a trade or business by the non-U.S. holder within the United States (and, if required by an applicable income tax treaty, is attributable to a permanent establishment, or fixed base in the case of an individual non-U.S. holder, that is maintained by the non-U.S. holder in the United States);

• the non-U.S. holder is an individual who is present in the United States for 183 days or more in the taxable year of that disposition, and certain other conditions are met; or

S-19

• we are or have been a “United States real property holding corporation” for U.S. federal income tax purposes at any time during the shorter of the five-year period ending on the date of such disposition and the non-U.S. holder’s holding period in our common stock.

A non-U.S. holder described in the first or second bullet point above generally will be subject to U.S. federal income tax on the net gain derived from the sale or disposition under regular graduated U.S. federal income tax rates as if the holder were a U.S. person. If the non-U.S. holder is a corporation, then the gain may also, under certain circumstances, be subject to the “branch profits” tax, which was discussed above.

With respect to the third bullet point, although there can be no assurance, we believe we are not, have not been and will not become a “United States real property holding corporation” for U.S. federal income tax purposes. In the event that we are or become a United States real property holding corporation at any time during the applicable period described in the third bullet point above, any gain recognized on a sale or other taxable disposition of our common stock may be subject to U.S. federal income tax, including any applicable withholding tax, if (i) the non-U.S. holder beneficially owns, or has owned, more than 5% of our common stock at any time during the applicable period, or (ii) our common stock ceases to be regularly traded on an “established securities market” within the meaning of the Code. Non-U.S. holders who intend to acquire more than 5% of our common stock are encouraged to consult their tax advisors with respect to the U.S. tax consequences of a disposition of our common stock.

Any proceeds from the disposition of our common stock will also be subject to the discussion below under the section titled “Foreign Account Tax Compliance Act Considerations.”

Federal Estate Tax

Common stock owned or treated as owned by an individual who is a non-U.S. holder at the time of his or her death generally will be included in the individual’s gross estate for U.S. federal estate tax purposes and may be subject to U.S. federal estate tax unless an applicable estate tax treaty provides otherwise.

Information Reporting and Backup Withholding

Information returns will be filed with the IRS in connection with payments of dividends on our common stock and the proceeds from a sale or other disposition of our common stock. Copies of information returns may be made available to the tax authorities of the country in which a non-U.S. holder resides or is incorporated under the provisions of a specific treaty or agreement.

You may be subject to backup withholding with respect to dividends paid on our common stock or with respect to proceeds received from a disposition of the shares of our common stock. Certain holders (including, among others, corporations and certain tax-exempt organizations) are generally not subject to backup withholding. You will be subject to backup withholding if you are not otherwise exempt and you

• fail to furnish your taxpayer identification number, or TIN, which, for an individual, is ordinarily his or her social security number;

• furnish an incorrect TIN;

• are notified by the IRS that you have failed to properly report payments of interest or dividends; or

• fail to certify, under penalties of perjury, that you have furnished a correct TIN and that the IRS has not notified you that you are subject to backup withholding.

Backup withholding is not an additional tax, but rather is a method of tax collection. You generally will be entitled to credit any amounts withheld under the backup withholding rules against your U.S. federal income tax liability provided that the required information is furnished to the IRS in a timely manner.

A non-U.S. holder may have to comply with certification procedures to establish that it is not a U.S. person in order to avoid information reporting and backup withholding tax requirements. The certification procedures required to claim a reduced rate of withholding under an income tax treaty will satisfy the certification requirements necessary to avoid backup withholding as well. The amount of any backup withholding from a payment to a non-U.S. holder may be allowed as a credit against such holder’s U.S. federal income tax liability and may entitle such non-U.S. holder to a refund, provided that the required information is timely furnished to the IRS.

S-20

Foreign Account Tax Compliance Act Considerations

The Foreign Account Tax Compliance Act, or FATCA, generally imposes a U.S. federal withholding tax at a rate of 30% on payments of dividends on, and gross proceeds from the sale or other disposition of, our common stock if paid to a foreign entity unless (i) if the foreign entity is a "foreign financial institution," the foreign entity must enter into an agreement with the IRS or, in the case of a foreign financial institution in a jurisdiction that has entered into an intergovernmental agreement with the United States, comply with the requirements of such agreement and undertake certain due diligence, reporting, withholding, and certain certification obligations, (ii) if the foreign entity is not a "foreign financial institution," the foreign entity identifies certain of its U.S. investors, if any, or (iii) the foreign entity is otherwise exempt under FATCA.

Under applicable U.S. Treasury Regulations, withholding under FATCA applies to payments of dividends on our common stock and will apply to payments of gross proceeds from a sale or other disposition of our common stock made on or after January 1, 2019. Under certain circumstances, a non-U.S. holder may be eligible for refunds or credits of the tax. An intergovernmental agreement between the United States and an applicable foreign country may modify the requirements described in this section. Non-U.S. holders should consult their own tax advisors regarding the possible implications of these rules on their investment in our common stock and the entities through which they hold our common stock, including, without limitation, the process and deadlines for meeting the applicable requirements to prevent the imposition of the 30% withholding tax under FATCA.

S-21

PLAN OF DISTRIBUTION

We have entered into an Equity Distribution Agreement with Piper Jaffray as our sales agent, which we filed as an exhibit to our Current Report on Form 8-K on July 27, 2017 and incorporate by reference in this prospectus supplement and the accompanying prospectus. Piper Jaffray will use commercially reasonable efforts to sell on our behalf all of our common stock requested to be sold by us, consistent with its normal trading and sales practices, under the terms and subject to the conditions set forth in the Equity Distribution Agreement. We may instruct Piper Jaffray not to sell our common stock if the sales cannot be effected at or above the price designated by us in any instruction or placement notice we deliver to Piper Jaffray We or Piper Jaffray may suspend the offering of our common stock upon proper notice and subject to other conditions, as further described in the Equity Distribution Agreement.

Piper Jaffray will provide written confirmation to us following the close of trading on the NASDAQ Global Market each day in which our shares are sold under the Equity Distribution Agreement. Each such confirmation will include the number of common stock sold on such day, the net proceeds to us, the compensation payable by us to Piper Jaffray in connection with the sales of such common stock, and the additional expenses deducted from the net proceeds payable to us.

We will pay Piper Jaffray commissions for its services in acting as agent in the sale of our common stock. Piper Jaffray will be entitled to compensation in an amount equal to 3.00% of the gross sales price of all of our common stock sold through it as agent under the Equity Distribution Agreement. We estimate that the total expenses for the offering, excluding compensation and out-of-pocket expenses payable to Piper Jaffray under the terms of the Equity Distribution Agreement, will be approximately $200,000. We have also agreed to reimburse Piper Jaffray for the reasonable out-of-pocket expenses and disbursements of its legal counsel, not related to any fees and expenses incurred in connection with the qualification of the shares or incident to any required review and approval by FINRA, in an amount not to exceed $50,000.

Settlement for sales of our common stock will occur on the third business day following the date on which any sales are made, or on some other date that is agreed upon by us and Piper Jaffray in connection with a particular transaction, in return for payment of the net proceeds to us. There is no arrangement for funds to be received in an escrow, trust or similar arrangement.

We will report at least quarterly the number of common stock sold through Piper Jaffray, as sales agent, under the Equity Distribution Agreement, the net proceeds to us and the compensation paid by us to Piper Jaffray in connection with the sales of our common stock.

Piper Jaffray and its affiliates have provided, and may in the future provide, various investment banking, commercial banking, fiduciary and advisory services for us from time to time for which they have received, and may in the future receive, customary fees and expenses. Piper Jaffray and its affiliates may, from time to time, engage in other transactions with and perform services for us in the ordinary course of their business.

In connection with the sale of our common stock on our behalf, Piper Jaffray may, and will with respect to sales effected in an “at-the-market” equity offering, be deemed to be an “underwriter” within the meaning of the Securities Act, and the compensation of Piper Jaffray may be deemed to be underwriting commissions or discounts. We have agreed to indemnify Piper Jaffray against specified liabilities, including liabilities under the Securities Act, or to contribute to payments that Piper Jaffray may be required to make because of those liabilities.

The offering of shares of our common stock pursuant to the Equity Distribution Agreement will terminate upon the earlier of (1) the sale of all shares subject to the Equity Distribution Agreement or (2) termination of the Equity Distribution Agreement. The Equity Distribution Agreement may be terminated by Piper Jaffray or us at any time on the close of business on the date of receipt of written notice, and by Piper Jaffray at any time in certain circumstances, including any suspension or limitation on the trading of our common stock on the NASDAQ Global Market, as further described in the Equity Distribution Agreement.

S-22

LEGAL MATTERS

Certain legal matters relating to this offering will be passed upon for us by Stradling Yocca Carlson & Rauth, P.C., Newport Beach, California. Certain legal matters relating to this offering will be passed upon for Piper Jaffray by Clifford Chance US LLP, New York, New York.

EXPERTS

The financial statements of Tandem Diabetes Care, Inc. at December 31, 2016 and 2015, and for each of the three years in the period ended December 31, 2016, have been incorporated by reference into this prospectus in reliance upon the reports of Ernst & Young LLP, independent registered public accounting firm, which reports are incorporated by reference into this prospectus (which contains an explanatory paragraph describing conditions that raise substantial doubt about the Company’s ability to continue as a going concern as described in Note 1 to the financial statements).

S-23

INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE

The SEC allows us to “incorporate” into this prospectus supplement and the accompanying prospectus information that we file with the SEC in other documents. This means that we can disclose important information to you by referring to other documents that contain that information. Any information that we incorporate by reference into this prospectus supplement and the accompanying prospectus is considered part of this prospectus supplement and the accompanying prospectus.

Information contained in this prospectus supplement and the accompanying prospectus, and information that we file with the SEC in the future and incorporate by reference in this prospectus supplement and the accompanying prospectus automatically modifies and supersedes previously filed information, including information in previously filed documents or reports that have been incorporated by reference in this prospectus supplement and the accompanying prospectus, to the extent the new information differs from or is inconsistent with the old information. Any statement so modified will be deemed to constitute a part of this prospectus supplement and the accompanying prospectus only as so modified, and any statement so superseded will be deemed not to constitute a part of this prospectus supplement or the accompanying prospectus.

We incorporate by reference, as of their respective dates of filing, the documents listed below that we have filed with the SEC and any documents that we file with the SEC pursuant to Sections 13(a), 13(c), 14 or 15(d) of the Securities Exchange Act of 1934, as amended, between the date of this prospectus supplement and the termination of the offering of the securities described in this prospectus supplement, other than, in each case, documents or information deemed to have been “furnished” and not “filed” in accordance with SEC rules:

• our Annual Report on Form 10-K for the year ended December 31, 2016, as filed with the SEC on March 8, 2017;

• our Definitive Proxy Statement on Schedule 14A, as filed with the SEC on April 6, 2017 (but only with respect to information required by Part III of our Annual Report on Form 10-K for the year ended December 31, 2016);

• our Quarterly Report on Form 10-Q for the period ended March 31, 2017, as filed with the SEC on April 27, 2017, and our Quarterly Report on Form 10-Q for the period ended June 30, 2017, as filed with the SEC on July 27, 2017;

• our Current Reports on Form 8-K filed on February 1, 2017, March 8, 2017, March 23, 2017, March 28, 2017, May 18, 2017, June 16, 2017, and July 18, 2017; and

• the description of our common stock contained in our registration statement on Form 8-A as filed with the SEC on November 8, 2013, as updated or amended in any amendment or report filed for such purpose.

You may request a free copy of any of the documents incorporated by reference into this prospectus. Requests should be made to:

David B. Berger, Esq.

General Counsel

Tandem Diabetes Care, Inc.

11045 Roselle Street

San Diego, California 92121

(858) 366-6900

Exhibits to the filings will not be sent, however, unless those exhibits have specifically been incorporated by reference in this prospectus supplement and the accompanying prospectus.

You should rely only on the information contained in this prospectus supplement, the accompanying prospectus, any document incorporated by reference herein or therein, or any free writing prospectuses we may provide to you in connection with this offering. Neither we nor Piper Jaffray have authorized anyone to provide you with any different information. We take no responsibility for, and can provide no assurance as to the reliability of, any other information that others may provide to you.

The information contained in this prospectus supplement, the accompanying prospectus, and in the documents incorporated by reference herein or therein is accurate only as of the date such information is presented. Our business, financial condition, results of operations and prospects may have changed since that date.

S-24

WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports and other information with the SEC. You may read and copy any document we file at the SEC’s Public Reference Room located at 100 F Street, N.E., Washington, D.C. 20549. You may obtain information on the operation of the public reference room by calling the SEC at 1-800-SEC-0330. Our filings with the SEC also are available from the SEC's internet site at http://www.sec.gov, which contains reports, proxy and information statements, and other information regarding issuers that file electronically.

This prospectus supplement and the accompanying prospectus are part of a registration statement that we filed with the SEC. As permitted by SEC rules, this prospectus supplement and the accompanying prospectus form a part of the registration statement, but do not contain all of the information that is included in the registration statement. The registration statement contains more information regarding us and our securities, including certain exhibits. You can obtain a copy of the registration statement from the SEC at the address listed above or from the SEC’s website.

S-25

$125,000,000

TANDEM DIABETES CARE, INC.

Common Stock

Preferred Stock

Warrants

Units

We may offer and sell, from time to time in one or more offerings, any combination of common stock, preferred stock, warrants or units having an aggregate initial offering price not exceeding $125,000,000. The warrants may be exercisable for shares of our common stock, shares of our preferred stock, and/or units.

This prospectus provides a general description of the securities we may offer. Each time we sell a particular class or series of securities, we will provide specific terms of the securities offered in a supplement to this prospectus. The prospectus supplement may also add, update or change information in this prospectus. You should read this prospectus and any prospectus supplement, as well as the documents incorporated by reference or deemed to be incorporated by reference herein or therein, carefully before you invest in any of the securities offered pursuant to this prospectus.

This prospectus may not be used to offer or sell our securities unless accompanied by a prospectus supplement relating to the offered securities.

These securities may be sold directly by us, through dealers or agents designated from time to time, to or through underwriters or dealers or through a combination of these methods on a continuous or delayed basis. For additional information on the methods of sale, you should refer to the section entitled “Plan of Distribution” in this prospectus. We will describe the plan of distribution for any particular offering of our securities in a prospectus supplement. If any agents, underwriters or dealers are involved in the sale of any securities with respect to which this prospectus is being delivered, we will set forth in a prospectus supplement the names of such agents or underwriters and any applicable fees, commissions, discounts and over-allotment options. We will also set forth in a prospectus supplement the price to the public of such securities and the net proceeds that we expect to receive from such sale.

Our common stock is listed on the NASDAQ Global Market and traded under the symbol “TNDM.” On December 1, 2014, the last reported sale price for our common stock on the NASDAQ Global Market was $13.29 per share.

INVESTING IN OUR SECURITIES INVOLVES A HIGH DEGREE OF RISK. YOU SHOULD REVIEW CAREFULLY THE RISKS AND UNCERTAINTIES DESCRIBED UNDER THE HEADING “RISK FACTORS” BEGINNING ON PAGE 4 OF THIS PROSPECTUS, AS WELL AS THE RISKS AND UNCERTAINTIES DESCRIBED UNDER A SIMILAR HEADING IN ANY APPLICABLE PROSPECTUS SUPPLEMENT AND IN THE DOCUMENTS THAT WE INCORPORATE BY REFERENCE HEREIN OR THEREIN.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the accuracy or adequacy of this prospectus. Any representation to the contrary is a criminal offense.

The date of this prospectus is December 19, 2014

TABLE OF CONTENTS

	Page
ABOUT THIS PROSPECTUS	1
SPECIAL NOTE REGARDING FORWARD-LOOKING INFORMATION	2
ABOUT THE COMPANY	3
RISK FACTORS	4
USE OF PROCEEDS	5
THE SECURITIES WE MAY OFFER	6
DESCRIPTION OF CAPITAL STOCK	7
DESCRIPTION OF WARRANTS	9
DESCRIPTION OF UNITS	10
PLAN OF DISTRIBUTION	11
LEGAL MATTERS	13
EXPERTS	13
INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE	14
WHERE YOU CAN FIND MORE INFORMATION	15

i

ABOUT THIS PROSPECTUS

This prospectus is part of a registration statement that we filed with the Securities and Exchange Commission, or the SEC, using a “shelf” registration process. Under this shelf registration process, we may from time to time offer and sell any combination of the securities described in this prospectus in one or more offerings with an aggregate initial offering price not to exceed $125,000,000. We have provided to you in this prospectus a general description of the securities we may offer. Each time we sell any of our securities under this prospectus, we will, to the extent required by law, provide a prospectus supplement that will contain specific information about the terms of the offering.

We may add, update or change any of the information contained in this prospectus or in any accompanying prospectus supplement we may authorize to be delivered to you. To the extent there is a conflict between the information contained in this prospectus and any accompanying prospectus supplement, you should rely on the information in the prospectus supplement, provided that if any statement in one of these documents is inconsistent with a statement in another document having a later date—for example, a document incorporated by reference in this prospectus or any prospectus supplement—the statement in the document having the later date modifies or supersedes the earlier statement. Any statement so modified will be deemed to constitute a part of this prospectus only as so modified, and any statement so superseded will be deemed not to constitute a part of this prospectus. This prospectus, together with any accompanying prospectus supplement, includes all material information relating to an offering pursuant to this registration statement.

You should rely only on the information contained in this prospectus, in any accompanying prospectus supplement, or in any document incorporated by reference herein or therein. We have not authorized anyone to provide you with any different information. We take no any responsibility for, and can provide no assurance as to the reliability of, any other information that others may provide to you. The information contained in this prospectus, in any applicable prospectus supplement, and in the documents incorporated by reference herein or therein is accurate only as of the date such information is presented. Our business, financial condition, results of operations and prospects may have changed since that date.

This prospectus and any accompanying prospectus supplement does not constitute an offer to sell or the solicitation of an offer to buy any securities other than the registered securities to which they relate, nor does this prospectus and any accompanying prospectus supplement constitute an offer to sell or the solicitation of an offer to buy securities in any jurisdiction to any person to whom it is unlawful to make such offer or solicitation in such jurisdiction. This prospectus may not be used to offer or sell our securities unless accompanied by a prospectus supplement relating to the offered securities.

This prospectus does not contain all of the information included in the registration statement. For a more complete understanding of the offering of the securities, you should refer to the registration statement, including its exhibits. The registration statement containing this prospectus, including the exhibits to the registration statement, provides additional information about us and the securities offered pursuant to this prospectus. The registration statement, including the exhibits, can be read on the SEC’s website or at the SEC’s offices mentioned under the heading “Where You Can Find More Information.”

1

SPECIAL NOTE REGARDING FORWARD-LOOKING INFORMATION

This prospectus, any accompanying prospectus supplement, and the documents incorporated by reference herein and therein, contain forward-looking statements within the meaning of the federal securities laws. These forward-looking statements are intended to qualify for the safe harbor from liability established by the Private Securities Litigation Reform Act of 1995. All statements other than statements of historical fact included in this prospectus, any accompanying prospectus supplement, or the documents incorporated by reference herein or therein, are forward looking statements.

Our forward-looking statements are based on our management’s current assumptions and expectations of future events and trends, which affect or may affect our business, strategy, operations or financial performance. Although we believe that these forward-looking statements are based upon reasonable assumptions, they are subject to numerous known and unknown risks and uncertainties and are made in light of information currently available to us. Many important factors, in addition to the factors described in this prospectus, may adversely and materially affect our results as indicated in forward-looking statements. You should read this prospectus, any accompanying prospectus supplement, and the documents we incorporate by reference herein and therein, completely and with the understanding that our actual future results may be materially different and worse from what we expect.

Moreover, we operate in an evolving environment. New risk factors and uncertainties emerge from time to time and it is not possible for our management to predict all risk factors and uncertainties, nor can we assess the impact of all factors on our business or the extent to which any factor, or combination of factors, may cause actual results to differ materially from those contained in any forward-looking statements. We qualify all of our forward-looking statements by these cautionary statements.  

Forward-looking statements speak only as of the date they were made, and, except to the extent required by law or the rules of the NASDAQ Stock Market, we undertake no obligation to update or review any forward-looking statement because of new information, future events or other factors. You should, however, review the risks and uncertainties we describe in the reports we will file from time to time with the SEC, after the date of this prospectus. See the information included under the heading “Where You Can Find More Information.”

Forward-looking statements involve risks and uncertainties and are not guarantees of future performance. As a result of the risks and uncertainties described above, the forward-looking statements discussed in this prospectus might not occur and our future results and our performance may differ materially from those expressed in these forward-looking statements due to, but not limited to, the factors mentioned above. Because of these uncertainties, you should not place undue reliance on these forward-looking statements when making an investment decision.

2

ABOUT THE COMPANY

Business

Tandem Diabetes Care^®, Inc. (the “Company”, “we”, “us” or “our”) is a medical device company with an innovative approach to the design, development and commercialization of products for people with insulin-dependent diabetes. We designed and commercialized our flagship product, the t:slim^® Insulin Delivery System, or t:slim, based on our proprietary technology platform and unique consumer-focused approach. Our technology platform features our patented Micro-Delivery™ Technology, a miniaturized pumping mechanism which draws insulin from a flexible bag within the pump’s cartridge rather than relying on a syringe and plunger mechanism. It also features an easy-to-navigate software architecture, a vivid color touchscreen and a micro-USB connection that supports both a rechargeable battery and t:connect^®, our data management application. Our innovative approach to product design and development is also consumer-focused and based on our extensive market research as we believe the user is the primary decision maker when purchasing an insulin pump. This research has consisted of more than 6,000 responses obtained in interviews, focus groups and online surveys, to understand what people with diabetes, their caregivers and healthcare providers are seeking in order to improve diabetes therapy management. We also apply the science of human factors to our design and development process, which seeks to optimize our devices to the intended users, allowing users to successfully operate our devices in their intended environment. Leveraging our technology platform and consumer-focused approach, we develop products to address unmet needs of people in all segments of the large and growing insulin-dependent diabetes market.

We developed t:slim to offer the specific features that people with insulin-dependent diabetes seek in a next-generation insulin pump. We designed it to have the look and feel of a modern consumer electronic device, such as a smartphone. It is the first and only insulin pump to feature a high resolution, color touchscreen. It is also the slimmest and smallest durable insulin pump currently on the market, and can easily and discreetly fit into a pocket, while still carrying a cartridge with 300 units of insulin. The touchscreen and intuitive software architecture make it easy to use, learn and teach, and to update the software without requiring any hardware changes. Similar to modern consumer electronic devices, t:slim incorporates colors, language, icons and feedback that consumers find intuitive to use. We offer a broad range of accessories allowing users to customize t:slim to their individual lifestyle and sense of style.

We have derived nearly all of our revenue from the sale of t:slim and associated supplies in the United States and expect to continue to do so until we are able to commercialize our other products that are currently under development. Other products that are currently under development include the t:slim G4™ Insulin Pump System, for which we submitted a pre-market approval, or PMA, application to the FDA in July 2014, and the t:flex™ Insulin Pump, for which we submitted a 510(k) application to the FDA in November 2014.

The t:slim G4 Insulin Pump System is designed to incorporate the same pump technology and user interface as t:slim and integrates our product platform with the DexCom, Inc^®. G4^® PLATINUM Continuous Glucose Monitor, or CGM. The integration provides a user the added convenience of allowing CGM information to be displayed on the pump. The t:flex Insulin Pump is also designed to include the same pump technology and user interface as t:slim, but will offer a larger, 480 unit insulin cartridge, compared to t:slim’s 300 unit cartridge.

Our headquarters and our manufacturing facility are located in San Diego, California.

Corporate Information

We were incorporated in Colorado in January 2006 and reincorporated in Delaware in January 2008. Our principal executive offices are located at 11045 Roselle Street, San Diego, California 92121. The telephone number of our principal executive office is (858) 366-6900. Our website is www.tandemdiabetes.com. The contents of our website are not incorporated by reference into this prospectus and should not be considered to be a part of this prospectus or relied upon in connection herewith.

3

RISK FACTORS

Investing in our securities involves a high degree of risk. Before making an investment decision, you should carefully consider the risks described in the sections entitled “Risk Factors” in our most recent Annual Report on Form 10-K and subsequent Quarterly Reports on Form 10-Q, as filed with the SEC, which are incorporated by reference in this prospectus in their entirety, as well as any amendment or updates to our risk factors reflected in subsequent filings with the SEC, including any applicable prospectus supplement. For more information, see “Where You Can Find More Information.” Our business, financial condition or results of operations could be materially adversely affected, and the trading price of our securities could decline as a result of, any of these risks.

The risks and uncertainties we have described are not the only ones we face. Additional risks and uncertainties not presently known to us or that we currently deem immaterial may also affect our business, financial condition or results of operations.

This prospectus and the documents we incorporate by reference in this prospectus contain forward-looking statements that involve risks and uncertainties. Our actual results could differ materially from those anticipated in these forward-looking statements as a result of certain factors, including the risks mentioned elsewhere in this prospectus. For more information, see “Special Note Regarding Forward-Looking Information.”

4

USE OF PROCEEDS

We intend to use the net proceeds we receive from the sale of our securities and from the exercise of the warrants issued pursuant hereto, if any, for working capital and other general corporate purposes. We may set forth additional information regarding the use of proceeds from the sale of securities we offer under this prospectus in a prospectus supplement relating to the specific offering. We have not determined the amount of net proceeds to be used specifically for the foregoing purposes. As a result, our management will have broad discretion in the allocation of the net proceeds.

Pending the use of the net proceeds, we intend to invest the net proceeds in high-quality, short-term interest-bearing obligations, investment-grade instruments, certificates of deposit or direct or guaranteed obligations of the U.S. government.

5

THE SECURITIES WE MAY OFFER

We may offer and sell, from time to time in one or more offerings, any combination of common stock, preferred stock, warrants, and units having an aggregate initial offering price not exceeding $125,000,000. The warrants may be exercisable for shares of our common stock, shares of our preferred stock, and/or units. In this prospectus, we refer to the common stock, preferred stock, warrants and units that we may offer collectively as “securities.”  

This prospectus provides a general description of the securities we may offer. Each time we sell any of our securities under this prospectus, we will, to the extent required by law, provide a prospectus supplement that will contain specific information about the terms of the offering. The prospectus supplement may also add, update or change information in this prospectus. For more information, see “About this Prospectus.”

6

DESCRIPTION OF CAPITAL STOCK

The following is a summary of all material characteristics of our capital stock as set forth in our amended and restated certificate of incorporation and amended and restated bylaws. The summary does not purport to be complete and is qualified in its entirety by reference to our amended and restated certificate of incorporation and amended and restated bylaws, copies of which have been filed as exhibits to our previous SEC filings. For more information, see “Where You Can Find More Information.”

Common Stock

We may issue shares of our common stock from time to time. We are authorized to issue 100,000,000 shares of common stock, par value $0.001 per share. As of November 30, 2014, there were 23,633,816 shares of common stock issued and outstanding.

Dividend Rights. Subject to preferences that may apply to any shares of preferred stock outstanding at the time, the holders of outstanding shares of our common stock are entitled to receive dividends out of funds legally available at the times and in the amounts that our board of directors may determine.

Voting Rights. Holders of our common stock are entitled to one vote per share. We have not provided for cumulative voting for the election of directors in our amended and restated certificate of incorporation. The board of directors is divided into three classes, which are as nearly equal in number as possible. Each director is elected for a three-year term with one class being elected at each year’s annual meeting of stockholders.

No Preemptive or Similar Rights. Our common stock is not entitled to preemptive rights, and is not subject to redemption. There are no sinking fund provisions applicable to our common stock.

Conversion. Our common stock is not convertible into any other shares of our capital stock.

Right to Receive Liquidation Distributions. Upon our liquidation, dissolution, distribution of assets or winding-up, the assets legally available for distribution to our stockholders would be distributable ratably among the holders of our common stock and any participating preferred stock outstanding at that time, if any, after payment of liquidation preferences, if any, on any outstanding shares of preferred stock and payment of claims of creditors.

Fully Paid and Non-Assessable. All of the outstanding shares of our common stock are, and the shares of our common stock issuable pursuant to this prospectus will be, fully paid and non-assessable.

Preferred Stock

As of November 30, 2014, no shares of our preferred stock were outstanding. Pursuant to the terms of our amended and restated certificate of incorporation, our board of directors is authorized, subject to limitations prescribed by Delaware law, to issue up to 5,000,000 shares of preferred stock, par value $0.001 per share, in one or more series, to establish from time to time the number of shares to be included in each series, and to fix the designation, powers, preferences and rights of the shares of each series and any of its qualifications, limitations or restrictions, in each case without further action by our stockholders. Our board of directors also can increase or decrease the number of shares of any series of preferred stock, but not below the number of shares of that series then outstanding. Our board of directors may authorize the issuance of preferred stock with voting or conversion rights that could adversely affect the voting power or other rights of the holders of our common stock. The issuance of preferred stock, while providing flexibility in connection with possible acquisitions and other corporate purposes, could, among other things, have the effect of delaying, deferring or preventing a change in our control or the removal of management and could adversely affect the market price of our common stock and the voting and other rights of the holders of our common stock.

Delaware Law and Certain Certificate of Incorporation and Bylaw Provisions

The provisions of Delaware law, our amended and restated certificate of incorporation and our amended and restated bylaws may have the effect of delaying, deferring or discouraging another person from acquiring control of us by means of a tender offer, a proxy contest or otherwise, or removing incumbent officers and directors. These provisions, summarized below, are expected to discourage certain types of coercive takeover practices and takeover bids that our board of directors may consider inadequate and to encourage any person seeking to acquire control of us to first negotiate with our board of directors.

7

Delaware Law. We are governed by the provisions of Section 203 of the Delaware General Corporation Law, or DGCL. In general, Section 203 prohibits a public Delaware corporation from engaging in a “business combination” with an “interested stockholder” for a period of three years after the date such stockholder became an “interested stockholder.” A “business combination” includes mergers, asset sales or other transactions resulting in a financial benefit to the stockholder. An “interested stockholder” is a person who, together with affiliates and associates, owns, or within three years did, prior to the determination of interested stockholder status, own, 15% or more of the corporation’s outstanding voting stock. These provisions may have the effect of delaying, deferring or preventing a change in control of our company not approved in advance by our board of directors.

Certificate of Incorporation and Bylaw Provisions. Our amended and restated certificate of incorporation and our amended and restated bylaws include a number of other provisions that could deter hostile takeovers or delay or prevent changes in control of our management team, including the following:

• Classified Board. Our amended and restated certificate of incorporation and amended and restated bylaws provide that our board is classified into three classes of directors. This could delay a successful tender offeror from obtaining majority control of our board of directors, and the prospect of that delay might deter a potential offeror.

• Stockholder Action; Special Meeting of Stockholders. Our amended and restated certificate of incorporation eliminates the right of stockholders to act by written consent. Our amended and restated certificate of incorporation further provides that special meetings of our stockholders may be called only by a majority of our board of directors.

• Advance Notice Requirements for Stockholder Proposals and Director Nominations. Our amended and restated certificate of incorporation and amended and restated bylaws provide advance notice procedures for stockholders seeking to bring business before our annual meeting of stockholders, or to nominate candidates for election as directors at our annual meeting of stockholders. These provisions might preclude our stockholders from bringing matters before our annual meeting of stockholders or from making nominations for directors at our annual meeting of stockholders.

• Amendment. Our amended and restated certificate of incorporation and our amended and restated bylaws provide that the affirmative vote of the holders of at least 66 2/3% of our voting stock then outstanding is required to amend certain provisions.

• Size of Board and Vacancies. Our amended and restated certificate of incorporation and amended and restated bylaws provide that the number of directors on our board of directors is fixed exclusively by our board of directors. Newly created directorships resulting from any increase in our authorized number of directors, and any vacancies resulting from death, resignation, retirement, disqualification, removal from office or other cause, will generally be filled by a majority of our board of directors then in office.

• Issuance of Undesignated Preferred Stock. Our board of directors will have the authority, without further action by our stockholders, to issue up to 5,000,000 shares of undesignated preferred stock with rights and preferences, including voting rights, designated from time to time by our board of directors. Our board of directors may utilize such shares for a variety of corporate purposes.

• No Cumulative Voting. The DGCL provides that stockholders are denied the right to cumulate votes in the election of directors unless our amended and restated certificate of incorporation provides otherwise. Our amended and restated certificate of incorporation does not provide for cumulative voting.

NASDAQ Global Market

Our common stock is listed on the NASDAQ Global Market and traded under the symbol “TNDM.” On December 1, 2014, the last reported sale price for our common stock on the NASDAQ Global Market was $13.29 per share.

Transfer Agent and Registrar

The transfer agent and registrar for our common stock is American Stock Transfer & Trust Company, LLC. The address of American Stock Transfer & Trust Company is 6201 15th Avenue, Brooklyn, NY 11219 and the telephone number is (718) 921-8200.

8

DESCRIPTION OF WARRANTS

We may issue warrants for the purchase of shares of common stock, shares of preferred stock, and/or units from time to time. We may issue warrants independently or together with common stock and/or preferred stock, and the warrants may be attached to or separate from those securities. If we issue warrants, they will be evidenced by warrant agreements or warrant certificates issued under one or more warrant agreements, which will be contracts between us and the holders of the warrants or an agent for the holders of the warrants. We encourage you to read the prospectus supplement that relates to any warrants we may offer, as well as the complete warrant agreement or warrant certificate that contain the terms of the warrants. If we issue warrants, the forms of warrant agreements and warrant certificates, as applicable, relating to the warrants will be filed as exhibits to the registration statement that includes this prospectus, or as an exhibit to a filing with the SEC that is incorporated by reference into this prospectus.

9

DESCRIPTION OF UNITS

We may issue units comprised of one or more of the other securities described in this prospectus in any combination from time to time. Each unit will be issued so that the holder of the unit is also the holder of each security included in the unit. Thus, the holder of a unit will have the rights and obligations of a holder of each included security. If we issue units, they will be evidenced by unit agreements or unit certificates issued under one or more unit agreements, which will be contracts between us and the holders of the units or an agent for the holders of the units. The unit agreement under which a unit is issued may provide that the securities included in the unit may not be held or transferred separately, at any time or at any time before a specified date. We encourage you to read the prospectus supplement that relates to any units we may offer, as well as the complete unit agreement or unit certificate that contain the terms of the units. If we issue units, the forms of unit agreements and unit certificates, as applicable, relating to the units will be filed as exhibits to the registration statement that includes this prospectus, or as an exhibit to a filing with the SEC that is incorporated by reference into this prospectus.

10

PLAN OF DISTRIBUTION

We may sell our securities from time to time in any manner permitted by the Securities Act of 1933, as amended, or the Securities Act, including any one or more of the following ways:

• through agents;

• to or through underwriters;

• to or through broker-dealers (acting as agent or principal);

• in “at the market” offerings, within the meaning of Rule 415(a)(4) of the Securities Act, to or through a market maker or into an existing trading market, on an exchange or otherwise; and/or

• directly to purchasers, through a specific bidding or auction process or otherwise.

The securities may be sold at a fixed price or prices, which may be changed, at market prices prevailing at the time of sale, at prices relating to the prevailing market prices or at negotiated prices.

Offers to purchase offered securities may be solicited by agents designated by us from time to time. Any agent involved in the offer or sale of the offered securities in respect of which this prospectus is delivered will be named, and any commissions payable by us will be set forth, in the applicable prospectus supplement. Unless otherwise set forth in the applicable prospectus supplement, any agent will be acting on a reasonable best efforts basis for the period of its appointment. Any agent may be deemed to be an underwriter, as that term is defined in the Securities Act, of the offered securities so offered and sold.

We will set forth in a prospectus supplement the terms of the offering of our securities, including:

• the name or names of any agents, underwriters or dealers;

• the purchase price of our securities being offered and the proceeds we will receive from the sale;

• any over-allotment options under which underwriters may purchase additional securities from us;

• any agency fees or underwriting discounts and commissions and other items constituting agents’ or underwriters’ compensation;

• the public offering price;

• any discounts or concessions allowed or reallowed or paid to dealers; and

• any securities exchanges on which such securities may be listed.

If offered securities are sold to the public by means of an underwritten offering, either through underwriting syndicates represented by managing underwriters or directly by the managing underwriters, we will execute an underwriting agreement with an underwriter or underwriters, and the names of the specific managing underwriter or underwriters, as well as any other underwriters, will be set forth in the applicable prospectus supplement. In addition, the terms of the transaction, including commissions, discounts and any other compensation of the underwriters and dealers, if any, will be set forth in the applicable prospectus supplement, which prospectus supplement will be used by the underwriters to make resales of the offered securities. If underwriters are utilized in the sale of the offered securities, the offered securities will be acquired by the underwriters for their own account and may be resold from time to time in one or more transactions, including:

• transactions on the NASDAQ Global Market or any other organized market where the securities may be traded;

• in the over-the-counter market;

• in negotiated transactions; or

• under delayed delivery contracts or other contractual commitments.

11

We may grant to the underwriters options to purchase additional offered securities to cover over-allotments, if any, at the public offering price with additional underwriting discounts or commissions, as may be set forth in the applicable prospectus supplement. If we grant any over-allotment option, the terms of the over-allotment option will be set forth in the applicable prospectus supplement.

We may authorize agents or underwriters to solicit offers by certain types of institutional investors to purchase securities from us at the public offering price set forth in the prospectus supplement pursuant to delayed delivery contracts providing for payment and delivery on a specified date in the future. We will describe the conditions to these contracts and the commissions we must pay for solicitation of these contracts in the prospectus supplement.

We may indemnify agents, underwriters and dealers against specified liabilities, including liabilities incurred under the Securities Act, or to contribution by us to payments they may be required to make in respect of such liabilities. Agents, underwriters or dealers, or their respective affiliates, may be customers of, engage in transactions with or perform services for us or our respective affiliates, in the ordinary course of business.

Unless otherwise specified in the applicable prospectus supplement, each class or series of securities will be a new issue with no established trading market, other than our common stock, which is traded on the NASDAQ Global Market. We may elect to list any other class or series of securities on any exchange and, in the case of our common stock, on any additional exchange. However, unless otherwise specified in the applicable prospectus supplement, we will not be obligated to do so. It is possible that one or more underwriters may make a market in a class or series of securities, but the underwriters will not be obligated to do so and may discontinue any market making at any time without notice. We cannot give any assurance as to the liquidity of the trading market for any of the offered securities.

Any underwriter may engage in over-allotment, stabilizing transactions, short-covering transactions and penalty bids in accordance with Regulation M under the Securities Exchange Act of 1934, as amended. Over-allotment involves sales in excess of the offering size, which create a short position. Stabilizing transactions permit bids to purchase the underlying security so long as the stabilizing bids do not exceed a specified maximum price. Syndicate-covering or other short-covering transactions involve purchases of the securities, either through exercise of the over-allotment option or in the open market after the distribution is completed, to cover short positions. Penalty bids permit the underwriters to reclaim a selling concession from a dealer when the securities originally sold by the dealer are purchased in a stabilizing or covering transaction to cover short positions. Those activities may cause the price of the securities to be higher than it would otherwise be. If commenced, the underwriters may discontinue any of the activities at any time.

To comply with the securities laws of certain states, if applicable, the securities offered by this prospectus will be offered and sold in those states only through registered or licensed brokers or dealers.

In compliance with guidelines of the Financial Industry Regulatory Authority, or FINRA, the maximum consideration or discount to be received by any FINRA member or independent broker dealer may not exceed 8% of the aggregate amount of the securities offered pursuant to this prospectus and any applicable prospectus supplement.

12

LEGAL MATTERS

Certain legal matters, including the validity of the issuance of the securities offered by this prospectus, will be passed upon for us by Stradling Yocca Carlson & Rauth, P.C., Newport Beach, California.

EXPERTS

Ernst & Young LLP, independent registered public accounting firm, has audited our financial statements included in our Annual Report on Form 10-K for the year ended December 31, 2013, as set forth in their report, which is incorporated by reference in this prospectus and elsewhere in the registration statement. Our financial statements are incorporated by reference in reliance on Ernst & Young LLP’s report, given on their authority as experts in accounting and auditing.

13

INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE

The SEC allows us to “incorporate” into this prospectus information that we file with the SEC in other documents. This means that we can disclose important information to you by referring to other documents that contain that information. Any information that we incorporate by reference into this prospectus is considered part of this prospectus.

Information contained in this prospectus and information that we file with the SEC in the future and incorporate by reference in this prospectus automatically modifies and supersedes previously filed information, including information in previously filed documents or reports that have been incorporated by reference in this prospectus, to the extent the new information differs from or is inconsistent with the old information. Any statement so modified will be deemed to constitute a part of this prospectus only as so modified, and any statement so superseded will be deemed not to constitute a part of this prospectus.

We incorporate by reference, as of their respective dates of filing, the documents listed below that we have filed with the SEC and any additional documents that we may file in the future with the SEC pursuant to Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act, including any documents filed after the date of the initial registration statement of which this prospectus is a part until the offering of the security covered by this prospectus has been completed, other than, in each case, documents or information deemed to have been “furnished” and not “filed” in accordance with SEC rules:

• our Annual Report on Form 10-K for the year ended December 31, 2013, as filed with the SEC on March 6, 2014;

• our Definitive Proxy Statement on Schedule 14A, as filed with the SEC on April 8, 2014;

• our Quarterly Reports on Form 10-Q for the fiscal quarter ended March 31, 2014, as filed with the SEC on May 6, 2014, for the fiscal quarter ended June 30, 2014, as filed with the SEC on July 31, 2014 and for the fiscal quarter ended September 30, 2014, as filed with the SEC on November 6, 2014;

• our Current Reports on Form 8-K as filed with the SEC on each of January 10, 2014, January 21, 2014, February 7, 2014, March 6, 2014, April 9, 2014, May 23, 2014 and June 20, 2014, and;

• the description of our common stock contained in our registration statement on Form 8-A as filed with the SEC on November 8, 2013, as updated or amended in any amendment or report filed for such purpose.

We will provide to each person, including any beneficial owner, to whom a prospectus is delivered, upon oral or written request, a copy of any document incorporated by reference. Requests should be made to:

David B. Berger, Esq.

General Counsel

Tandem Diabetes Care, Inc.

11045 Roselle Street

San Diego, California 92121

(858) 366-6900

You should rely only on the information contained in this prospectus, in any accompanying prospectus supplement, or in any document incorporated by reference herein or therein. We have not authorized anyone to provide you with any different information. We take no any responsibility for, and can provide no assurance as to the reliability of, any other information that others may provide to you. The information contained in this prospectus, in any applicable prospectus supplement, and in the documents incorporated by reference herein or therein is accurate only as of the date such information is presented. Our business, financial condition, results of operations and prospects may have changed since that date.

14

WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports and other information with the SEC. You may read and copy any document we file at the SEC's Public Reference Room located at 100 F Street, N.E., Washington, D.C. 20549. You may obtain information on the operation of the public reference room by calling the SEC at 1-800-SEC-0330. Our filings with the SEC also are available from the SEC's internet site at http://www.sec.gov, which contains reports, proxy and information statements, and other information regarding issuers that file electronically.

This prospectus is part of a registration statement that we filed with the SEC. As permitted by SEC rules, this prospectus and any accompanying prospectus supplement that we may file, which form a part of the registration statement, do not contain all of the information that is included in the registration statement. The registration statement contains more information regarding us and our securities, including certain exhibits. You can obtain a copy of the registration statement from the SEC at the address listed above or from the SEC’s website.

15

Up to $15,000,000

TANDEM DIABETES CARE, INC.

Common Stock

P R O S P E C T U S  S U P P L E M E N T

Piper Jaffray

July 27, 2017