The SU-122 (from Samokhodnaya Ustanovka 122 mm) was a Soviet self-propelled howitzer used during World War II. The number "122" in the designation represents the caliber of the main armament—a 122 mm M-30S howitzer.
The Soviet High Command became interested in assault guns following the success of German Sturmgeschutz III SPGs. Assault guns had some advantages over tanks. The lack of a turret made them cheaper to produce. They could be built with a larger fighting compartment and could be fitted with bigger and more powerful weapons on a given chassis. However, assault guns could only aim their weapons by turning the entire vehicle, and were thus less suited for close combat than tanks.
In April 1942, design bureaus were asked to develop several assault guns with various armament: 76.2 mm ZiS-3 divisional field guns and 122 mm M-30 howitzers for infantry support, and 152 mm ML-20 howitzers for breaking through enemy strongholds.
A prototype assault gun, armed with the 122 mm howitzer and built on the German Sturmgeschütz III chassis was designated SG-122. Only 10 of these were completed. Production was halted when the vehicle was judged to be unsuccessful and found to be hard to maintain.