HMS Seadog was a third-batch S-class submarine built for the Royal Navy during World War II.

[6] On 22 September 1942, Seadog, under the command of Lieutenant Anthony Daniel, sailed to Holy Loch, where she was commissioned into the Royal Navy two days later.

[8] Between November 1942 and February 1943, Seadog conducted three patrols off Norway, protecting Arctic convoys to and from Northern Russia, but did not sight any potential targets.

Returning from these operations, the submarine docked in Ardrossan to have a 20-millimetre (0.8 in) Oerlikon light anti-aircraft gun added aft of the conning tower and additional equipment installed.

On 10 June, Seadog obtained an ASDIC contact with a U-boat and blindly launched a torpedo in its direction, but missed; it may have been the German submarine U-417, which was sunk the next day by aircraft.

[7] The submarine conducted another patrol in the Arctic, taking part in Operation Corncrake, picking up Norwegian commandos on the island of Spitsbergen, and missed another German U-boat.

On 13 August, the submarine stopped and boarded the small French fishing vessel St. Moquet, interrogated its crew, and examined its papers.

[7] Between mid-September and early December 1943, Seadog conducted three patrols off Norway, but was unsuccessful in spotting targets; during her first, she landed a relief force on Spitsbergen.

The submarine departed on another patrol on 24 December, operating off Stadlandet, Norway, meeting more luck—after four days at sea, she sank the German transport Oldenburg with a full salvo of six torpedoes; a depth charge counter-attack by the ship's escort followed, but did not cause damage.

[7] On her next patrol, the submarine sank a coaster with torpedoes off Ulèë Lheuë, Sumatra, and a sailing vessel near Sigli.

Three days later, Imperial Japan announced it would surrender, and Seadog was sent back to Great Britain, passing through Suez and Gibraltar, and arriving on 18 October.