Donald K. 'Deke' Slayton (March 1, 1924–June 13, 1993) was an American astronaut. Selected as one of the of Mercury 7, he was the only one who did not fly during the Mercury program. Slayton had been scheduled to fly in 1962 on the on the second orbital flight, but due to an erratic heart rate (idiopathic atrial fibrillation), his place was taken by Scott Carpenter.

A US Air Force pilot, Slayton resigned his commission in 1963 and worked for NASA in a civilian capacity as head of Astronaut selection. In this capacity he had the decisive role of choosing the crews for the Gemini and Apollo programmes including the decision of who would be the first man on the moon. Slayton remained extremely loyal to the other Mercury astronauts who remained in the space programme ensuring they were given assignments.

A long medical program led to him being restored to full flight status in 1973, when he was chosen as docking module pilot for the Apollo-Soyuz Test Project, designed to allow a docking between the American Apollo spacecraft and the Soyuz spacecraft of the Soviet Union. On July 17, 1975, the two craft joined up in orbit, and astronauts Slayton, Thomas Stafford and Vance D. Brand conducted crew transfers with cosmonauts Aleksey A. Leonov and Valeriy Kubasov.