The S-class or Srednyaya submarines were part of the Soviet Navy's underwater fleet during World War II. Unofficially nicknamed Stalinets, boats of this class were the most successful and achieved the most significant victories among all Soviet submarines. In all, they sank 82,770 gross register tons (GRT) of merchant shipping and seven warships, which accounts for about one-third of all tonnage sunk by Soviet submarines during the war.

The Leninets or L class were the second class of submarines to be built for the Soviet Navy. Twenty-five were built in four groups between 1931 and 1941. They were minelaying submarines and were based on the British L-class submarine, HMS L55, which was sunk during the British intervention in the Russian Civil War. Some experience from the previous Dekabrist-class submarines was also utilised. The boats were of the saddle tank type and mines were carried in two stern galleries as pioneered on the pre-war Russian submarine Krab (1912). These boats were considered successful by the Soviets. Groups 3 and 4 had more powerful engines and a higher top speed.