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The SAAB 21 is a Swedish single-seat low-wing monoplane fighter and attack aircraft designed and manufactured by SAAB. It used a relatively unorthodox twin boom fuselage with a pusher engine, giving the aircraft an unusual appearance.

The first example from the first batch of 54 of the first production variant, the J 21A-1 was delivered to the Swedish Air Force on 1 December 1945.[5] Construction was at SAAB's main plant in Trollhättan and deliveries of this model ran until 5 December 1946, when deliveries of two batches totalling 124 of the J 21A-2 began. A third order, in two batches totalling 119 of the A21A-3 fighter-bomber completed production of the piston-engine variants.[13] A total of 298 J 21As were constructed prior to the production line ending in 1948.

Despite the original intention for the type to be principally in air defence roles, in service, the J 21 was utilized mainly in the light bomber role. The type was used only by the Swedish Air Force.

Officials doubted its effectiveness due to its unconventional design.[9] In response, the Swedish Air Board requested that SAAB study a development with the engine in the nose as the J 23.[9] This aircraft used the J 21's DB 605B engine and had a more traditional appearance, similar to the North American P-51 Mustang, but its projected performance was reduced, which became a key factor in improving the J 21 instead. The Swedish Air Force became interested in jet propulsion and from 1945, SAAB began studying modifications of the airframe to accommodate a jet engine in place of its piston engine.[9] Production of the piston-engine version continued until 1948 while examples of the new jet engine version began being converted on the line. As a result of the conversions, the piston-engine powered J 21A began being retired in 1954.