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Soyuz 7K-T No.39, (also named Soyuz 18a or Soyuz 18-1) was an unsuccessful launch of a manned Soyuz spacecraft by the Soviet Union on April 5, 1975. The mission was expected to dock with the orbiting Salyut 4 space station, but due to a failure of the Soyuz launch vehicle the crew failed to achieve orbit.

The accident was the result of a failure of a rocket staging event; the core booster of the Soyuz rocket did not separate from its upper stage. Since the accident took place after the escape tower had jettisoned, the Soyuz 7K-T spacecraft needed to use its own propulsion module engines to escape the failing rocket.

The escape exerted excessive g forces on the crew, consisting of commander Vasili Lazarev, an Air Force major, and flight engineer Oleg Makarov, a civilian. Both cosmonauts were injured, with Lazarev suffering injuries serious enough to end his career. The descent module landed near Aleysk, in the Altai Mountains; the crew initially feared they landed in the People's Republic of China, leading them to burn their paperwork in case they were captured by the Chinese, whom the Soviet Union were at odds with at the time.

The accident was disclosed by the normally secretive Soviets, as it occurred during preparations for their joint Apollo–Soyuz Test Project with the United States three months later. This would prove to be the last manned Soyuz mission launched with the original Soyuz rocket; future missions would be launched by the updated Soyuz-U rocket.