The Twenty-first Amendment (Amendment XXI) to the United States Constitution repealed the Eighteenth Amendment to the United States Constitution, which had mandated nationwide prohibition on alcohol. The Twenty-first Amendment was proposed by the 72nd Congress on February 20, 1933, and was ratified by the requisite number of states on December 5, 1933. It is unique among the 27 amendments of the U.S. Constitution for being the only one to repeal a prior amendment, as well as being the only amendment to have been ratified by state ratifying conventions.

The Eighteenth Amendment was ratified on January 16, 1919, the result of years of advocacy by the temperance movement. The subsequent enactment of the Volstead Act established federal enforcement of the nationwide prohibition on alcohol. As many Americans continued to drink despite the amendment, Prohibition gave rise to a profitable black market for alcohol, fueling the rise of organized crime. Throughout the 1920s, Americans increasingly came to see Prohibition as unenforceable, and a movement to repeal the Eighteenth Amendment grew until the Twenty-first Amendment was ratified in 1933.

Section 1 of the Twenty-first Amendment expressly repeals the Eighteenth Amendment. Section 2 bans the importation of alcohol into states and territories that have laws prohibiting the importation or consumption of alcohol. Several states continued to be "dry states" in the years after the repeal of the Eighteenth Amendment, but in 1966 the last dry state (Mississippi) legalized the consumption of alcohol. Nonetheless, several states continue to closely regulate the distribution of alcohol. Many states delegate their power to ban the importation of alcohol to counties and municipalities, and there are numerous dry communities throughout the United States. Section 2 has occasionally arisen as an issue in Supreme Court cases that touch on the Commerce Clause.