SEC Form 424B3 filed by Nuveen California Select Tax-Free Income Portfolio

Nuveen California Select Tax-Free Income Portfolio (NYSE: NXC) (the “Fund”)

Supplement Dated October 31, 2024

To The Fund’s Currently Effective Prospectus

Effective October 31, 2024, the information appearing under the heading “Risk Factors” of the Prospectus is hereby supplemented with the following:

Direct lending risk: The Fund may engage in direct lending. Direct loans between the Fund and a borrower may not be administered by an underwriter or agent bank. The Fund may provide financing to commercial borrowers directly or through companies affiliated with the Fund. The terms of the direct loans are negotiated with borrowers in private transactions. Furthermore, a direct loan may be secured or unsecured. The Fund will rely primarily upon the creditworthiness of the borrower and/or any collateral for payment of interest and repayment of principal. Direct loans may subject the Fund to liquidity risk, interest rate risk, and borrower default or insolvency. Direct loans are not publicly traded and may not have a secondary market which may have an adverse impact on the ability of the Fund to dispose of a direct loan and/or value the direct loan. The Fund’s performance may be impacted by the Fund’s ability to lend on favorable terms as the Fund may be subject to increased competition or a reduced supply of qualifying loans which could lead to lower yields and reduce Fund performance.

As part of its lending activities, the Fund may originate loans to companies that are experiencing significant financial or business difficulties, including companies involved in bankruptcy or other reorganization and liquidation proceedings. Although the terms of such financing may result in significant financial returns to the Fund, they involve a substantial degree of risk. The level of analytical sophistication, both financial and legal, necessary for successful financing to companies experiencing significant business and financial difficulties is unusually high. Different types of assets may be used as collateral for the Fund’s loans and, accordingly, the valuation of and risks associated with such collateral will vary by loan. There is no assurance that the Fund will correctly evaluate the value of the assets collateralizing the Fund’s loans or the prospects for a successful reorganization or similar action. In any reorganization or liquidation proceeding relating to a borrower that the Fund is lending money to, the Fund may lose all or part of the amounts advanced to the borrower or may be required to accept collateral with a value less than the amount of the loan advanced by the Fund to the borrower. Furthermore, in the event of a default by a borrower, the Fund may have difficulty disposing of the assets used as collateral for a loan. To the extent the Fund seeks to engage in direct lending, the Fund will be subject to enhanced risks of litigation, regulatory actions and other proceedings. As a result, the Fund may be required to pay legal fees, settlement costs, damages, penalties or other charges, any or all of which could materially adversely affect the Fund and its holdings.

PLEASE KEEP THIS WITH YOUR

FUND’S PROSPECTUS FOR FUTURE REFERENCE

EGN-NXCP-1024P