Gemini 9A was a 1966 crewed spaceflight in NASA's Gemini program. It was the seventh crewed Gemini flight, the 15th crewed American flight and the 23rd spaceflight of all time. The original crew for Gemini 9, command pilot Elliot See and pilot Charles Bassett, were killed in a crash on February 28, 1966, while flying a T-38 jet trainer to the McDonnell Aircraft plant in St. Louis, Missouri to inspect their spacecraft. Their deaths promoted the backup crew, Thomas P. Stafford and Eugene Cernan, to the prime crew. The mission was renamed Gemini 9A after the original May 17 launch was scrubbed when the mission's Agena Target Vehicle was destroyed after a launch failure. The mission was flown June 3–6, 1966, after launch of the backup Augmented Target Docking Adaptor (ATDA). Stafford and Cernan rendezvoused with the ATDA, but were unable to dock with it because the nose fairing had failed to eject from the docking target due to a launch preparation error. Cernan performed a two-hour extravehicular activity, during which it was planned for him to demonstrate free flight in a self-contained rocket pack, the USAF Astronaut Maneuvering Unit. He was unable to accomplish this due to stress, fatigue, and overheating.

Project Gemini was the second United States human spaceflight program to fly. Conducted after the first American manned space program, Project Mercury, while the Apollo program was still in early development, Gemini was conceived in 1961 and concluded in 1966. The Gemini spacecraft carried a two-astronaut crew. Ten Gemini crews and 16 individual astronauts flew low Earth orbit (LEO) missions during 1965 and 1966.