The Eighteenth Amendment (Amendment XVIII) of the United States Constitution established the prohibition of alcohol in the United States. The amendment was proposed by Congress on December 18, 1917, and was ratified by the requisite number of states on January 16, 1919. The Eighteenth Amendment was repealed by the Twenty-first Amendment on December 5, 1933. It is the only amendment to be repealed.

The Eighteenth Amendment was the product of decades of efforts by the temperance movement, which held that a ban on the sale of alcohol would ameliorate poverty and other societal issues. The Eighteenth Amendment declared the production, transport, and sale of intoxicating liquors illegal, though it did not outlaw the actual consumption of alcohol. Shortly after the amendment was ratified, Congress passed the Volstead Act to provide for the federal enforcement of Prohibition. The Volstead Act declared that liquor, wine, and beer all qualified as intoxicating liquors and were therefore prohibited. Under the terms of the Eighteenth Amendment, Prohibition began on January 17, 1920, one year after the amendment was ratified.

Although the Eighteenth Amendment led to a decline in alcohol consumption in the United States, nationwide enforcement of Prohibition proved difficult, particularly in cities. Rum-running (bootlegging) and speakeasies became popular in many areas. Public sentiment began to turn against Prohibition during the 1920s, and 1932 Democratic presidential nominee Franklin D. Roosevelt called for its repeal. The Twenty-first Amendment finally did repeal the Eighteenth in 1933, making the Eighteenth Amendment the only one so far to be repealed in its entirety.

The Temperance movement in the United States is a movement to curb the consumption of alcohol. It had a large influence on American politics and American society in the nineteenth and twentieth centuries, culminating in the unsuccessful prohibition of alcohol, through the Eighteenth Amendment to the United States Constitution, from 1920 to 1933. Today, there are organizations that continue to promote the cause of temperance.