Nuveen Select Tax-Free Income Portfolio (the Fund) is a closed-ended, investment company. The Fund seeks to provide current income and stable dividends, exempt from regular federal and designated state income taxes, where applicable, consistent with the preservation of capital by investing primarily in a portfolio of municipal obligations. The Fund invests all of its assets in long-term municipal obligations with investment-grade ratings (Baa and BBB or better). The Fund may invest up to 20% of net assets in unrated municipal obligations deemed to be of comparable quality. It may invest up to 20% of its net assets in municipal obligations that pay interest subject to the federal alternative minimum tax. The Fund invests in inverse floating rate securities. The Funds' investment adviser is Nuveen Fund Advisors, Inc. (Nuveen Fund Advisors), formerly Nuveen Asset Management.