The KV-1S ("Skorostnoy'" — "Speedy") was a Soviet heavy tank used during the Great Patriotic War. It was developed in 1942 by the design bureau of the Chelyabinsk Tractor Plant. The KV-1, which had been in service since 1940, offered good armour protection, but tank crews were dissatisfied with the unreliability of certain components and the tank's poor mobility. Consequently, the design bureau at the Chelyabinsk Tractor Plant carried out a development of a modernisation from May to July 1942. The tank's dimensions were reduced, its side armour was weakened and a new transmission was installed. These changes improved the vehicle's overall reliability and mobility. The tank was accepted into service as the KV-1S in August 1942 and immediately entered mass production. A total of 1,086 vehicles were built between 1942 and 1943 and used by the Red Army during the Great Patriotic War.

The KV-1S was introduced in Update 1.57 "Battle March". Compared to the KV-1 (L-11), its improved mobility allows it to advance alongside medium tanks and manoeuvre actively in battle. Its high reverse speed, which is unusual for Soviet tanks, enables it to retreat quickly behind cover. The 76 mm ZiS-5 cannon has a wide selection of ammunition, including the powerful BR-350B APCBC shell, which has good penetration capabilities. However, its reduced armour reduces the KV-1S's survivability, and its slow turret traverse and lengthy reload time are problematic during close-range engagements.