Soyuz 18 (Russian: Союз 18, Union 18) was a 1975 Soviet manned mission to Salyut 4, the second and final crew to man the space station. Pyotr Klimuk and Vitali Sevastyanov set a new Soviet space endurance record of 63 days and the mark for most people in space simultaneously (seven) was tied during the mission.

The Soyuz 18 crew were the back-up crew for the failed Soyuz 18a mission, carried out that mission's objectives, and continued the work of the previous Soyuz 17 crew. Klimuk and Sevastyanov were launched into space on 24 May 1975 and docked with Salyut 4 two days later. The crew quickly set to performing experiments and fixing or replacing equipment. A spectrometer was repaired, a gas analyzer was replaced, and a pumping condenser in the water regeneration system was switched with a hand pump.

On 29 and 30 May, biological and medical experiments were performed and the Oasis garden was started. Studies of the stars, planets, Earth and its atmosphere were started on 2 and 3 June. Some 2,000 photographs of the Earth and 600 of the sun were reported taken.

More medical experiments were performed in June, and attempts were made to grow plants, including onions. Experiments were carried out on insects, and experiments on varying the work schedule were carried out.

Extensive medical experiments were carried out on 23 June, as the crew surpassed the Soviet space endurance record of 29 days, set by Soyuz 17, the Salyut's previous crew. The all-time record was held then by the Skylab 4 crew at 84 days.

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