The Delfinen class was a trio of minelaying submarines operated by the Swedish Navy between 1936 and 1953. The boats were the first class of minelaying submarines in the Navy and were later used as the basis for further submarines developed immediately before and during World War II. They had an uneventful service history, and were all retired in 1953.
Following World War I, Swedish naval authorities were impressed by the performance of German minelaying submarines. As a result, they ordered the construction of , the first Swedish minelaying submarine. The design of Valen was followed up a decade later by the three Delfinen-class boats. Like their predecessors, the submarines were fitted with vertical tubes in the outer ballast tanks to launch naval mines. Design work was undertaken by NV Ingenieurskantoor voor Scheepsbouw, a clandestine Dutch front company intended to maintain German submarine experience to circumvent limitations placed by the Treaty of Versailles.
The design featured a length overall of , a beam of , and a draught of , with a complement of 34. Armament consisted of four torpedo tubes, three in the bow and one aft, along with a deck gun, one anti-aircraft gun, and up to 20 mines. They were powered by two MAN diesel engines and two electric motors, which developed and , respectively, which turned two propeller shafts. While surfaced, the class reached a top speed of and displaced 540 tons, and when submerged, they could make , and displaced 720 tons.
The three submarines were built by Kockums, and were all laid down in 1933. Each boat was commissioned by 1935, and were later used as the basis for further Swedish submarine designs. In 1936, the minelaying tubes were removed to produce the coastal submarines, and an additional stern torpedo tube was added to produce the minelaying submarine in 1942.