The SU-122 was designed as a successor to the SU-76 assault gun which despite being built in huge numbers, suffered from numerous teething troubles and inadequate armament. The SU-122 was also the first of the SU series of gun platforms to be based on the T-34 chassis, predating the SU-85/100 which were directly based on it in turn. As such, the SU-122 shared the T-34 characteristic sloped hull design but with the turret removed in favor of a superstructure located in the front of the hull. The superstructure, of low profile, was also fully enclosed, thereby resolving one of the main problems with the SU-76, namely its poor protection against anything beyond small arms fire. Furthermore, the large superstructure enabled it to be operated by a crew of five and the reliable T-34 chassis gave it excellent speed and mobility; enough to be able to keep up with the tanks. The SU-122 was armed with the 122mm M-30S gun which was used primarily for infantry support against enemy strongpoints, but its low muzzle velocity did not make it an effective tank-killer except at short-ranges, even when armed with HEAT rounds. The SU-122 was used in most Soviet offensive operations during 1943-44 but was discontinued from production in favor of the SU-100 tank destroyer and the SU-152 assault gun which was more effective in the infantry support role. Surviving examples remained in service briefly after the war.

Design of a new self-propelled gun mounted on a medium tank chassis began as early as April 1942, and the prototype U-35 was eventually ordered into production as the SU-122, the first of which were built in December 1942 for service the following year. There were no variants, but it served as the basis for the SU-85/100 tank destroyers that were developed later.