Soyuz 25 (, Union 25) was an October 1977 Soviet crewed space flight, the first to the new Salyut 6 space station, which had been launched 10 days earlier. However, the mission was aborted when cosmonauts Vladimir Kovalyonok and Valery Ryumin failed to engage the docking latches of the station despite five attempts. Lacking sufficient fuel to attempt a dock at the other end of the station and with battery power for only two days, they returned to Earth.
The failure led to a new rule whereby every crew had to have at least one person aboard who had previously flown in space. The crew were to stay on board for about 90 days, which would break the Soviet space endurance record, and possibly the all-time record of 84 days held by the crew of Skylab 4. The station was a new version of a Salyut with modifications including a second docking port which allowed supply ships and other crews to visit during the duration of a mission.