Soyuz 21 was a 1976 Soviet crewed mission to the Salyut 5 space station, the first of three flights to the station. The mission's objectives were mainly military in scope, but included other scientific work.

Salyut 5, the last dedicated military space station in the Soviet space program, was launched 22 June 1976. Its first crew was launched 14 days later on 6 July, with Commander Volynov and Flight Engineer Zholobov aboard Soyuz 21. They docked with the station the next day, and gave a televised tour 8 July.

On 24 August, it was announced the mission was to end in only 10 hours, a development which caught even the reporters of Radio Moscow by surprise. The reason for the sudden termination of the mission was reported at the time to have been an acrid odor that developed in the environmental control system. The problem was said to have begun as early as 17 August. Because Soyuz 21 was returning early, it was outside the normal recovery window. It then encountered strong winds as it descended, which caused uneven firing of the retrorockets. It made a hard landing around midnight 200 km southwest of Kokchetav, Kazakhstan.

This is Soyuz's 36th flight, and 20th crewed flight.

The Soyuz is a Soviet crewed spaceship, developed to made manned lunar missions. This version called 7K will fly 4 times on the giant launcher N1, and several tens of times on Proton to fly over the Moon, which will be successful during the mission Zond 4. Soyuz will become the first spacecraft to transport living beings to the Moon during the flight of Zond 5, with two turtles. Subsequently, it is adapted to low orbit and will fly on the Soyuz launcher to serve the Salyut and Mir stations and the ISS.