Soyuz 31 was a 1978 Soviet crewed space flight to the Salyut 6 space station. It was the seventh mission to and sixth successful docking at the orbiting facility. The Soyuz 31 crew were the second to visit the long-duration Soyuz 29 resident crew.

Soyuz 31 carried Valery Bykovsky and Sigmund Jähn, the first German cosmonaut, into space. They swapped Soyuz craft with the long-duration crew and returned to Earth in Soyuz 29, the resident crew returned to Earth in Soyuz 31.

This is Soyuz's 48th flight, and 30th 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.