Free Writing Prospectus to Preliminary Pricing Supplement No. 1,621

Registration Statement Nos. 333-275587; 333-275587-01

Dated March 29, 2024; Filed pursuant to Rule 433

Morgan Stanley

5-Year Worst-of INDU, NDX and RTY Jump Securities with Auto-Callable Feature

This document provides a summary of the terms of the securities. Investors must carefully review the accompanying preliminary pricing supplement referenced below, product supplement, index supplement and prospectus, and the “Risk Considerations” on the following page, prior to making an investment decision.

Terms
Issuing entity:	Morgan Stanley Finance LLC
Guarantor:	Morgan Stanley
Underlying indices:	Dow Jones Industrial AverageSM (“INDU”), NASDAQ-100 Index® (“NDX”) and Russell 2000® Index (“RTY”)
Early redemption:	Determination date:		Early redemption payment:
	1st:	5/1/2025		$1,125
Downside threshold level:	70% of the initial index value for each underlying index
Leverage factor:	165% to 180% (applicable only if the securities are not redeemed prior to maturity and the final index value of each underlying index is greater than the respective initial index value)
Pricing date:	April 30, 2024
Final determination date:	April 30, 2029
Maturity date:	May 3, 2029
CUSIP:	61776LMB0
Preliminary pricing supplement:	https://www.sec.gov/Archives/edgar/data/895421/000183988224009919/ms1621_424b2-06034.htm

1All payments are subject to our credit risk

Hypothetical Examples

Early Redemption1
Date	Change in Worst Performing Underlying Index	Payment (per security)
1st Determination Date	+20%	$1,125
The securities are automatically redeemed on the early redemption date. Investors will receive a payment of $1,125 per security on the early redemption date.

Hypothetical Payout at Maturity1
Assuming that one or more of the underlying indices close below the respective initial index value(s) on the first annual determination date, and, consequently, the securities are not automatically redeemed prior to, and remain outstanding until, maturity:
Change in Worst Performing Underlying Index	Payment (per security)
+30%	$1,495*
+20%	$1,330*
+10%	$1,165*
0%	$1,000
-10%	$1,000
-20%	$1,000
-30%	$1,000
-31%	$690
-40%	$600
-50%	$500
-70%	$300
-90%	$100
*Assumes a leverage factor of 165%

The issuer has filed a registration statement (including a prospectus) with the SEC for the offering to which this communication relates. Before you invest, you should read the prospectus in that registration statement and other documents the issuer has filed with the SEC for more complete information about the issuer and this offering. You may get these documents for free by visiting EDGAR on the SEC Web site at www.sec.gov. Alternatively, the issuer, any underwriter or any dealer participating in the offering will arrange to send you the prospectus if you request it by calling toll-free 1-800-584-6837.

Underlying Indices

For more information about the underlying indices, including historical performance information, see the accompanying preliminary pricing supplement.

Risk Considerations

The risks set forth below are discussed in more detail in the “Risk Factors” section in the accompanying preliminary pricing supplement. Please review those risk factors carefully prior to making an investment decision.

Risks Relating to an Investment in the Securities

●The securities do not pay interest or guarantee the return of any principal.

●If the securities are redeemed prior to maturity, the appreciation potential of the securities is limited by the fixed early redemption payment specified for the first determination date.

●The market price will be influenced by many unpredictable factors.

●The securities are subject to our credit risk, and any actual or anticipated changes to our credit ratings or credit spreads may adversely affect the market value of the securities.

●As a finance subsidiary, MSFL has no independent operations and will have no independent assets.

●The estimated value of the securities is approximately $926.00 per security, or within $55.00 of that estimate, and is determined by reference to our pricing and valuation models, which may differ from those of other dealers and is not a maximum or minimum secondary market price.

●Not equivalent to investing in the underlying indices.

●Reinvestment risk.

●The securities will not be listed on any securities exchange and secondary trading may be limited, and accordingly, you should be willing to hold your securities for the entire 5-year term of the securities.

●The rate we are willing to pay for securities of this type, maturity and issuance size is likely to be lower than the rate implied by our secondary market credit spreads and advantageous to us. Both the lower rate and the inclusion of costs associated with issuing, selling, structuring and hedging the securities in the original issue price reduce the economic terms of the securities, cause the estimated value of the securities to be less than the original issue price and will adversely affect secondary market prices.

●Hedging and trading activity by our affiliates could potentially adversely affect the value of the securities.

●The calculation agent, which is a subsidiary of Morgan Stanley and an affiliate of MSFL, will make determinations with respect to the securities.

●The U.S. federal income tax consequences of an investment in the securities are uncertain.

Risks Relating to the Underlying Indices

●You are exposed to the price risk of each underlying index.

●Adjustments to the underlying indices could adversely affect the value of the securities.

●The securities are linked to the Russell 2000® Index and are subject to risks associated with small-capitalization companies.

Tax Considerations

You should review carefully the discussion in the accompanying preliminary pricing supplement under the caption “Additional Information About the Securities – Tax considerations” concerning the U.S. federal income tax consequences of an investment in the securities, and you should consult your tax adviser.