The Agena Target Vehicle (; ATV), also known as Gemini-Agena Target Vehicle (GATV) was an uncrewed spacecraft used by NASA during its Gemini program to develop and practice orbital space rendezvous and docking techniques, and to perform large orbital changes, in preparation for the Apollo program lunar missions.

The spacecraft was based on Lockheed Aircraft's Agena-D upper stage rocket, fitted with a docking target manufactured by McDonnell Aircraft. The name 'Agena' derived from the star Beta Centauri, also known as Agena.

The combined spacecraft was a 26-foot (7.92 m)-long cylinder with a diameter of 5 feet (1.52 m), placed into low Earth orbit with the Atlas-Agena launch vehicle. It carried approximately 14,021 to 14,054 pounds (6,360 to 6,375 kg) of propellant and gas at launch, and had a gross mass at orbital insertion of 7,117 to 7,271 pounds (3,228 to 3,298 kg).

The ATV for Gemini 6 failed on launch on October 25, 1965, which led NASA to develop the Augmented Target Docking Adapter (ATDA) as a backup, a smaller spacecraft consisting of the docking target fitted with an attitude control propulsion system, but lacking the Agena orbital change rocket. The ATDA was used once on Gemini 9A after a second ATV launch failure on May 17, 1966, but failed as a docking target because its launch shroud failed to separate.

Gemini 8 (officially Gemini VIII) was the sixth crewed spaceflight in NASA's Gemini program. It was launched on March 16, 1966, and was the 14th crewed American flight and the 22nd crewed spaceflight overall. The mission conducted the first docking of two spacecraft in orbit, but also suffered the first critical in-space system failure of a US spacecraft which threatened the lives of the astronauts and required an immediate abort of the mission. The crew returned to Earth safely.

Flown by pilot David Scott and command pilot Neil Armstrong, the flight marked the second time a US civilian flew into space and the first time a US civilian flew into orbit.