The S-class or Srednyaya (Russian: Средняя, "medium") submarines were part of the Soviet Navy's underwater fleet during World War II.

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.

In the early 1930s the Soviet government started a massive program of general rearmament, including naval expansion.

The rnew Shchuka-class submarine was satisfactory, but it was designed for shallow Baltic Sea service and lacked true ocean-going capabilities.

After its defeat in World War I, the German Weimar Republic was forbidden under the terms of the Treaty of Versailles to have submarines or build them in its own yards.

One of these proxies, the Netherlands-based NV Ingenieurskantoor voor Scheepsbouw (IvS), a subsidiary of Deutsche Schiff- und Maschinenbau AG-AG Weser, was developing a submarine that matched Soviet requirements.

Several German naval officers (including Wilhelm Canaris) visited Spain and struck a deal with a Spanish businessman, Horacio Echevarrieta.

A submarine was built in 1929–1930, and was tested at sea early in 1931, under the manufacturer's designation of submarino E-1, since no navy had yet commissioned the ship.

Soviet engineers, among others, visited the yard in 1932 and were generally satisfied with the design, but suggested several modifications and improvements, in expectation of future local production.

Despite several problems encountered during the boat's trials, the design was considered satisfactory and the Soviet government bought it, with the condition Deschimag make the suggested improvements and assist with the building of several prototypes, which it did.

Construction of the first two prototypes commenced in December 1934 at the Baltic Shipyard (Baltiysky zavod) in Leningrad, using partially German equipment.

Initially the first prototypes received the names N-1, N-2, and N-3 (Nemetskaya, "German") but in October 1937 they were re-named S-x (Srednyaya, "Medium").

In the West, the class was much more widely known for its nickname, Stalinets, coined with reference to earlier boats of the Leninets type, but it was never featured in any official documents.

The third boat, while still using other German machinery, was powered by Soviet-made diesels, due to delays in delivery of originally intended ones.

During the war, the former riverboat production yard #638 in Astrakhan was employed for the completion of several boats constructed in Leningrad and Gorky.

The batteries lacked the traditional central walkway, instead using special service trolleys suspended from the deckhead.

The electrical system omitted the complicated layout common on earlier Soviet designs, and was simple and reliable.

The boats were equipped with two periscopes, observation PZ-7.5 and targeting PA-7.5, mounted very close to each other and reports existed of difficulties in using them simultaneously.