The RT-21M Pioneer was a medium-range ballistic missile with a nuclear warhead deployed by the Soviet Union from 1976 to 1988. It was withdrawn from service under the Intermediate-Range Nuclear Forces Treaty. Its NATO reporting name was SS-20 Saber and its bilateral identifier was RSD-10.

The missile was 16.5 m high, 1.79 m in diameter and weighed 37 tons. It was based on two solid-fuel fibre-glass clad stages of the RT-21 (SS-16 Sinner). The missile's range was from 600 to 5,000 km initially, the final model had a maximum range of, possibly, 7,500 km. Initially the missile was fitted with a single 1 MT, 1.6 ton warhead, later models could take one warhead or two and from 1980 three MIRV'd 150 KT devices. The CEP was also reduced from 550 metres to 450.

It was intended to replace, or augment, the R-12 (SS-4 Sandal) and R-14 (SS-5 Skean) missiles deployed from 1958 and 1961 respectively in the USSR and Warsaw Pact states. A design concept was approved in 1968 and the task given to the Moscow Institute of Thermal Technology and Alexander Nadiradze . Flight testing began in 1974 and deployment commenced on March 11, 1976, with the first supplied units becoming operational in August of that year. Up to 1986 a total of 48 launch sites were equipped with 405 SS-20 missiles.

The United States deployed Pershing missiles in Western Europe to counter the SS-20.

654 missiles were built in total; they and 499 associated mobile launchers were all destroyed by May 1991. Under agreement with the US fifteen missiles were preserved for museum or similar purposes.

See also: List of missiles

SS-20 is also the name of a number of punk bands from the 1980s, most notably from Poland (later renamed Dezerter ), Cincinnati, Ohio, and, perhaps most infamously, Mexico.