Soyuz 18a (Russian: Союз 18a, Union 18a) (also called Soyuz 18-1 and the April 5 Anomaly) was a manned Soyuz spacecraft launched by the Soviet Union in 1975, intended to dock with the orbiting Salyut 4 space station, but which failed to achieve orbit due to a serious malfunction during launch. The crew consisted of commander Vasili Lazarev, an Air Force major, and flight engineer Oleg Makarov, a civilian.

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 which flew three months later. The crew, who feared initially they had landed in China, were successfully recovered.

Soyuz 18a was intended to be the second mission to take cosmonauts to the Soviet Salyut 4 space station for a 60-day mission. Both cosmonauts were on their second mission and had flown their first mission together, Soyuz 12, in September 1973 to test a new type of Soyuz spacecraft after the fatal accident of Soyuz 11.