STS-51-J was the 21st NASA Space Shuttle mission and the first flight of Space Shuttle Atlantis. It launched from Kennedy Space Center, Florida, on October 3, 1985, carrying a payload for the U.S. Department of Defense, and landed at Edwards Air Force Base, California, on October 7.

STS-51-J launched on October 3, 1985, at 11:15 EDT, from Launch Pad 39A at the Kennedy Space Center. The launch was delayed by 22 minutes and 30 seconds due to a problem with a main engine liquid hydrogen prevalve close remote power controller; the controller was showing a faulty "on" indication.

The mission was the second shuttle flight totally dedicated to deploying a Department of Defense payload, after STS-51-C. Its cargo was classified, but it was reported that two (USA-11 and USA-12) DSCS-III (Defense Satellite Communications System) satellites were launched into stationary orbits by an Inertial Upper Stage. The DSCS satellites used X-band frequencies (8/7 GHz). Each DSCS-III satellite had a design life of ten years, although several of the DSCS satellites have far exceeded their design life expectancy.

The mission was deemed successful. After a flight lasting 4 days, 1 hour and 45 minutes, Atlantis landed on Runway 23 at Edwards Air Force Base at 13:00 EDT on October 7, 1985. During STS-51-J, mission commander Karol Bobko became the first astronaut to fly on three different shuttle orbiters, and the only astronaut to fly on the maiden voyages of two different orbiters.