Action T4 (German: Aktion T4) was the name used after World War II for Nazi Germany's "Euthanasia programme" during which physicians killed thousands of people who were "judged incurably sick, by critical medical examination". The programme officially ran from September 1939 until August 1941, but it continued unofficially until the end of the Nazi regime in 1945.

During the official stage of Action T4 70,273 people were killed, but the Nuremberg Trials found evidence that German and Austrian physicians continued the murder of patients after October 1941 and that about 275,000 people were killed under T4. More recent research based on files recovered after 1990 gives a figure of at least 200,000 physically or mentally handicapped people killed by medication, starvation, or in the gas chambers between 1939 and 1945.

The name T4 was an abbreviation of "Tiergartenstraße 4", the address of a villa in the Berlin borough of Tiergarten which was the headquarters of the Gemeinnützige Stiftung für Heil- und Anstaltspflege, bearing the euphemistic name literally translating into English as Charitable Foundation for Curative and Institutional Care. This body operated under the direction of Reichsleiter Philipp Bouhler, the head of Hitler's private chancellery, and Dr. Karl Brandt, Hitler's personal physician. This villa no longer exists, but a plaque set in the pavement on Tiergartenstraße marks its location.

Action may refer to: