Aktion T4 was a campaign of mass murder by involuntary euthanasia in Nazi Germany. The term was first used in post-war trials against doctors who had been involved in the killings. The name T4 is an abbreviation of Tiergartenstraße 4, a street address of the Chancellery department set up in early 1940, in the Berlin borough of Tiergarten, which recruited and paid personnel associated with Aktion T4. Certain German physicians were authorised to select patients "deemed incurably sick, after most critical medical examination" and then administer to them a "mercy death". In October 1939, Adolf Hitler signed a "euthanasia note", backdated to 1 September 1939, which authorised his physician Karl Brandt and Reichsleiter Philipp Bouhler to begin the killing.

Involuntary euthanasia, typically regarded as a type of murder, occurs when euthanasia is performed on a person who would be able to provide informed consent, but does not, either because they do not want to die, or because they were not asked.