UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D. C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

KNOT Offshore Partners LP

(Name of Issuer)

Common Units Representing Limited Partner Interests

(Title of Class of Securities)

Y48125 101

(CUSIP Number)

Knutsen NYK Offshore Tankers AS

Smedasundet 40

Postbox 2017

5504 Haugesund

Norway

with a copy to:

Catherine S. Gallagher

Vinson & Elkins L.L.P.

2200 Pennsylvania Avenue NW, Suite 500W

Washington, DC 20037

Telephone: (202) 639-6500

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

May 18, 2016

(Date of Event which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition which is the subject of this Schedule 13D, and is filing this schedule because of Rule 13d-1(e), 13d-1(f) or 13d-1(g), check the following box:  ¨

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of section 18 of the Securities Exchange Act of 1934 (the “Exchange Act”) or otherwise subject to the liabilities of that section of the Exchange Act but shall be subject to all other provisions of the Act (however, see the Notes).

(Continued on following pages)

SCHEDULE 13 D

The Reporting Persons named in Item 2 below are hereby jointly filing this Schedule 13D because, due to certain affiliations and relationships among the Reporting Persons, such Reporting Persons may be deemed to beneficially own some or all of the same securities of the Partnership. In accordance with Rule 13d-1(k)(1)(iii) promulgated pursuant to the Exchange Act, the Reporting Persons named in Item 2 below have executed a written agreement relating to the joint filing of this Schedule 13D (the “Joint Filing Agreement”), a copy of which is annexed hereto as Exhibit E.

This Schedule 13D relates to the common units representing limited partner interests (the “Common Units”) in KNOT Offshore Partners LP, a Marshall Islands limited partnership (the “Issuer” or the “Partnership”), which has its principal executive offices at 2 Queens Cross, Aberdeen, Aberdeenshire AB15 4YB, United Kingdom.

(a) This Schedule 13D is being filed by (i) Knutsen NYK Offshore Tankers AS, a Norway limited company (“KNOT”), (ii) NYK Logistics Holding (Europe) B.V., a Netherlands company (“NYK Europe”), (iii) Nippon Yusen Kabushiki Kaisha, a Japanese corporation (“NYK”), (iv) TS Shipping Invest AS, a Norway limited company (“TSSI”), (v) Seglem Holding AS, a Norway limited company (“Seglem Holding”) and (vi) Trygve Seglem, a citizen of Norway (collectively, the “Reporting Persons”).

KNOT owns 100% of KNOT Offshore Partners GP LLC, a Marshall Islands limited liability company and the general partner of the Partnership (the “General Partner”). KNOT is a joint venture between NYK Europe and TSSI, each of which owns a 50% interest and has the power to appoint half of the members of the board of directors of KNOT. NYK Europe is a wholly owned subsidiary of NYK, a broadly owned Japanese public company. TSSI is a wholly owned subsidiary of Seglem Holding, of which 70% is owned by Trygve Seglem with the remainder owned by members of his immediate family. Accordingly, each of NYK Europe, NYK, TSSI, Seglem Holding and Trygve Seglem may be deemed to share beneficial ownership of the Common Units of the Partnership held by KNOT and the 2.01% general partner interest in the Partnership held by the General Partner.

(b) The principal business address of each of KNOT, TSSI, Seglem Holding and Trygve Seglem is Smedasundet 40, Postbox 2017, 5504 Haugesund, Norway. The principal business address of NYK Europe is Herikerbergweg 238, Luna ArenA, 1101 CM Amsterdam, Netherlands. The principal business address of NYK is 3-2 Marunouchie 2-chome, Chiyoda-ku Toyko 100-005, Japan.

(c) The principal business of KNOT is to own and operate shuttle tankers. The principal business of TSSI and Seglem Holding is as an investment and holding company. The principal business of NYK Europe is financial holdings. The principal business of NYK is international marine transportation, terminal and harbor transport, and shipping-related services. The name, business address, present principal occupation and citizenship of Trygve Seglem and of each director and executive officer of the entity Reporting Persons (the “Covered Persons”) are set forth on Exhibit A to this Schedule 13D, which is incorporated herein by reference.

(d) Except for the disclosure contained in the following sentence, in the past five years, none of the Reporting Persons nor, to the best of their knowledge, any Covered Person has been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors). On March 11, 2015, NYK entered a guilty plea to one count of criminal price fixing pursuant to Section 1 of the Sherman Antitrust Act, and was fined $59.4 million by the United States District Court, District of Maryland.

(e) In the past five years, none of the Reporting Persons nor, to the best of their knowledge, any Covered Person has been a party to a civil proceeding of a judicial or administrative body of competent jurisdiction that has resulted in a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violations with respect to such laws.

(f) Trygve Seglem is a citizen of Norway.

Issuance of Subordinated Units at IPO

In connection with the consummation of the Partnership’s initial public offering (“IPO”), the Partnership acquired a 100% indirect ownership interest in KNOT Shuttle Tankers AS, a wholly owned subsidiary of KNOT, which at the time of the IPO directly or indirectly owned (1) 100% of Knutsen Shuttle Tankers XII KS, the owner of the shuttle tankers Recife Knutsen and Fortaleza Knutsen, (2) 100% of Knutsen Shuttle Tankers XII AS, the general partner of Knutsen Shuttle Tankers XII KS, and (3) the shuttle tankers Windsor Knutsen and Bodil Knutsen and all of their related charters, inventory and long-term debt. In connection with the consummation of the IPO, (1) the Partnership issued to KNOT 8,567,500 subordinated units, representing a 49.0% limited partner interest in the Partnership (“Subordinated Units”), and 100% of the incentive distribution rights; (2) the General Partner, a wholly owned subsidiary of KNOT, continued its 2.0% general partner interest in the Partnership; and (3) the Partnership issued and sold to the public, through certain underwriters, 8,567,500 Common Units, representing a 49.0% limited partner interest in the Partnership.

Conversion of Subordinated Units

The Partnership’s First Amended and Restated Agreement of Limited Partnership (the “Partnership Agreement”) provided that the Subordinated Units owned by KNOT would convert into Common Units two business days after the Partnership met certain financial tests set forth in the Partnership Agreement. These financial tests required the Partnership to have earned and paid a minimum quarterly distribution on all of the outstanding units for three consecutive, non-overlapping four-quarter periods. On May 16, 2016, the Partnership paid the final distribution required to satisfy the financial tests. On May 18, 2016, the 8,567,500 Subordinated Units converted into Common Units on a one-for-one basis and KNOT became the beneficial owner of 8,567,500 additional Common Units.

Unit Repurchase Program

In August 2015, the Partnership’s board of directors authorized a program for the Partnership to repurchase up to 666,667 Common Units. The board of directors of the General Partner concurrently authorized the General Partner to purchase up to 333,333 Common Units. As of August 18, 2016, the General Partner had purchased 90,368 Common Units pursuant to the unit repurchase program at an average purchase price of $12.71 per unit. The funds used by the General Partner to purchase such Common Units were obtained through equity contribution and long term borrowings from KNOT, which KNOT funded with available cash generated by its operations. Because KNOT is the sole owner of the General Partner, the 90,368 Common Units purchased by the General Partner pursuant to the unit repurchase program are included in the Common Units reported in this Schedule 13D as beneficially owned by KNOT. The press release of the Partnership announcing the unit repurchase program is incorporated by reference in Exhibit D.

The information contained in Item 3 above is incorporated herein by reference.

KNOT acquired its Common Units for investment purposes and (in the case of the original acquisition of the Subordinated Units) in order to effect the IPO. Each of the Reporting Persons at any time and from time to time may acquire additional Common Units or dispose of any or all of Common Units that it owns depending upon an ongoing evaluation of the investment in the Common Units, prevailing market conditions, other investment opportunities, other investment considerations and/or other factors.

Under the Limited Liability Company Agreement of the General Partner, dated February 20, 2013 (the “General Partner LLC Agreement”), KNOT has the right to designate the individuals that serve on the board of directors of the General Partner. The General Partner, in turn, has the right to appoint three of the seven directors of the Partnership. Through KNOT’s right to appoint the board of directors of the General Partner and the General

Partner’s right to appoint members of the board of directors of the Partnership, KNOT and, indirectly, the other Reporting Persons, have the ability to influence the management, policies and control of the Partnership with the aim of increasing the value of the Partnership, and thus of the Reporting Persons’ investment.

Except as disclosed herein, the Reporting Persons have no plans or proposals which relate to, or could result in, any of the matters referred to in paragraphs (a) through (j) inclusive of the instructions to Item 4 of Schedule 13D. The Reporting Persons may, at any time and from time to time, review or reconsider their position and/or change their purpose and/or formulate plans or proposals with respect thereto.

(a) – (b) The aggregate number and percentage of Common Units beneficially owned by each Reporting Person (on the basis of a total of 27,194,094 Common Units outstanding as of August 18, 2016) are as follows:

(c) Except as described herein, none of the Reporting Persons nor, to the Reporting Persons’ knowledge, the Covered Persons, has effected any transactions in the Common Units during the past 60 days.

(d) The Reporting Persons have the right to receive distributions from, and the proceeds from the sale of, the Common Units reported on the cover page of this Schedule 13D and in this Item 5. No other person is known by the Reporting Persons to have the right to receive or the power to direct the receipt of distributions from, or the proceeds from the sale of, Common Units beneficially owned by the Reporting Persons.

(e) Not applicable.

The information contained in Items 2 and 3 above is incorporated herein by reference.

Partnership Agreement

General Partner’s Right to Appoint Directors

The Partnership’s board of directors consists of seven members, three of which are appointed by the General Partner (“Appointed Directors”) and four of which are elected by holders of the Common Units (“Elected Directors”). Appointed Directors serve as directors for terms determined by General Partner. Elected Directors are divided into four classes serving staggered four-year terms

Voting Restrictions

The Partnership Agreement provides that holders of the Common Units who are residents of Norway (including KNOT) are not eligible to vote in the election of Elected Directors. Further, if any person or group owns beneficially more than 4.9% of any class of units then outstanding, any such units owned by that person or group in excess of 4.9% may not be voted on any matter and are not considered to be outstanding when sending notices of a meeting of unitholders, calculating required votes (except for purposes of nominating a person for election to the board of directors), determining the presence of a quorum or for other similar purposes, unless required by law. The General Partner, its affiliates (including KNOT) and persons who acquire Common Units with the prior approval of the board of directors are not subject to the 4.9% limitation except with respect to voting their Common Units in the election of the Elected Directors.

Registration Rights

Under the Partnership Agreement, the Partnership agreed to register for resale under the Securities Act of 1933, as amended, and applicable state securities laws any Common Units or other partnership interests proposed to be sold by the General Partner or any of its affiliates (including KNOT) or their assignees if an exemption from the registration requirements is not otherwise available or advisable. These registration rights continue for two years following any withdrawal or removal of the General Partner as the general partner of the Partnership. The Partnership is obligated to pay all expenses incidental to the registration, excluding underwriting discounts and commissions.

Limited Call Right

Under the Partnership Agreement, if, at any time, the General Partner and its affiliates own more than 80% of the Common Units then outstanding, the General Partner has the right, but not the obligation, to purchase all, but not less than all, of the remaining Common Units at a price equal to the greater of (1) the average of the daily closing prices of the Common Units over the consecutive 20 trading days preceding the date three days before notice of exercise of the call right is first mailed and (2) the highest price paid by the General Partner or any of its affiliates for Common Units during the 90-day period preceding the date such notice is first mailed.

General Partner LLC Agreement

Under the General Partner LLC Agreement, KNOT has the right to designate the individuals that serve on the board of directors of the General Partner.

To the best knowledge of the Reporting Persons, there are no other contracts, arrangements, understandings or relationships (legal or otherwise) among the persons named in Item 2 with respect to any securities of the Partnership.

References to, and descriptions of, the Partnership Agreement as set forth in this Schedule 13D are qualified in their entirety by reference to the Partnership Agreement filed as Exhibit 1.2 of the Partnership’s Annual Report on Form 20-F for the fiscal year ended December 31, 2013 filed with the U.S Securities and Exchange Commission (the “Commission”) on April 15, 2014, which is incorporated by reference herein in its entirety. References to, and descriptions of, the General Partner LLC Agreement as set forth in this Schedule 13D are qualified in their entirety by reference to the General Partner LLC Agreement filed as Exhibit 3.4 to the Partnership’s Registration Statement on Form F-1, filed with the Commission on February 28, 2013 (File No. 333-186947), which is incorporated by reference herein in its entirety.

Signature

After reasonable inquiry and to the best of their knowledge and belief, the undersigned certify that the information set forth in this statement is true, complete and correct.

Date: August 19, 2016

EXHIBIT A

Directors and Executive Officers of Knutsen NYK Offshore Tankers AS

Directors and Executive Officers of NYK Logistics Holding (Europe) B.V.

Directors and Executive Officers of Nippon Yusen Kabushiki Kaisha

Directors and Executive Officers of TS Shipping Invest AS

Directors and Executive Officers of Seglem Holding AS

EXHIBIT E

Joint Filing Agreement

The undersigned, being duly authorized thereunto, hereby execute this agreement as an exhibit to this Schedule 13D with respect to the Common Units of KNOT Offshore Partners LP to evidence the agreement of the below-named parties, in accordance with the rules promulgated pursuant to the Securities Exchange Act of 1934, as amended, to file this Schedule 13D jointly on behalf of each such party.