Soyuz 16 (Russian: Союз 16, Union 16) was a 1974 manned test flight for a joint Soviet-US space flight which culminated in the Apollo-Soyuz mission in July, 1975. The two-man Soviet crew tested a docking ring and other systems to be used in the joint flight.

In an unprecedented move, Soviet planners offered to inform their NASA counterparts of the time of the launch, as long as they did not reveal that time to the press. NASA officials refused to agree to that condition and, accordingly, were informed of the launch an hour after it occurred, on 2 December 1974.

During the flight, Cosmonauts Anatoly Filipchenko and Nikolai Rukavishnikov tested the androgynous docking system to be used for the ASTP mission by retracting and extending a simulated 20 kg American docking ring.The crew also tested modified environmental systems, new solar panels and improved control systems, as well as a new radar docking system. air pressure was reduced from 760 mm to 540 mm and oxygen raised from 20% to 40% to test reducing the planned transfer time to Apollo from two to one hour. On 7 December 1974, the docking ring was jettisoned with explosive bolts to test emergency measures if the capture latches got stuck during the ASTP flight.

The craft landed 8 December 1974, near Arkalyk and was hailed a complete success. The mission duration, six days, matched the ASTP mission duration to within 10 minutes.

Early concepts for a joint flight included docking a Soyuz craft to the American Skylab space station, or an Apollo vehicle docking with a Salyut space station. Once the Americans abandoned their Skylab station in 1974, the Apollo-Salyut concept seemed to be the logical choice, but since the Soviets had started to develop a universal docking adapter for the mission and feared having to publicly reveal details of their military-focused Salyut missions, the two powers opted to link a Soyuz spacecraft with an Apollo spacecraft.

Three test flights of an uncrewed version of the ASTP spacecraft were flown: Kosmos 638, launched 3 April 1974; Kosmos 652, launched 15 May 1974; and Kosmos 672, launched 12 August 1974. These three flights, and Soyuz 16, were all launched with an improved version of a Soyuz booster.