Katzenbach v. Morgan, 384 U.S. 641 (1966), was a landmark decision of the Supreme Court of the United States regarding the power of Congress, pursuant to Section 5 of the Fourteenth Amendment, to enact laws that enforce and interpret provisions of the Constitution.

Section 4(e) was aimed at securing the franchisement of New York City's large Puerto Rican population and "provides that no person who has completed the sixth grade in a public school, or an accredited private school, in Puerto Rico in which the language of instruction was other than English shall be disfranchised for inability to read or write English."

Justice Brennan applied the appropriateness standard of McCulloch v. Maryland (1819) to determine whether the legislation passed constitutional muster.

By striking down the state application of the Religious Freedom Restoration Act of 1993, it addressed the separation of powers concerns that had been voiced earlier by Harlan.

Christopher W. Schmidt, "Section 5’s Forgotten Years: Congressional Power to Enforce the Fourteenth Amendment Before Katzenbach v. Morgan," Northwestern University Law Review 113 (2018): 47–108.