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. The mission ended abruptly with cosmonauts Boris Volynov and Vitaly Zholobov returning to Earth after 49 days in orbit. The precise reason for the early end of the mission was the subject of much speculation, but was reported to be an emergency evacuation after the Salyut atmosphere developed an acrid odor.

Salyut 5, also known as OPS-3, was a Soviet space station. Launched in 1976 as part of the Salyut programme, it was the third and last Almaz space station to be launched for the Soviet military. Two Soyuz missions visited the station, each crewed by two cosmonauts. A third Soyuz mission attempted to visit the station, but failed to dock, whilst a fourth mission was planned but never launched.