Soyuz 31 (Russian: Союз 31, Union 31) was a 1978 Soviet manned 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.

Soyuz 31, the third Intercosmos flight, was launched 26 August 1978. Cosmonauts Bykovsky and Jahn were greeted by resident crew Vladimir Kovalyonok and Aleksandr Ivanchenkov when they docked at the aft port of the Salyut 6 space station the next day. The visitors brought with them fresh onions, garlic, lemons, apples and other food for the long-duration crew, then in space for more than two months.

The presence of the East German cosmonaut was seen as significant because of the presence of the MKF-6M camera on the space station, built by the Carl Zeiss works at Jena. Medical and biological experiments were carried out, including an audio experiment which tested sound and noise perception limits. An experiment called Berolina used the Splav furnace to process an ampoule of bismuth and antimonide with the material between two plates in the ampoule. The tree structure which resulted was four to six times larger than what had been produced on the ground. Another experiment tested using different photographic films on the station's interior.

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