was a member of the 2nd Submarine Flotilla. From 23 to 26 August 1939 the 2nd Submarine Flotilla moved to their war bases at Dundee and Blyth. On 1 September, the day the war started, Swordfish departed Dundee for her first war patrol, southwest of Stavanger, Norway. On 14 September, southeast of Aberdeen, Scotland in position 56°22'N, 01°28'W, she was mistaken for a U-boat by her sister ship , which fired three torpedoes. Swordfish dived and the torpedoes missed. On 23 September she left Dundee for her second patrol, west of Denmark. Informed the next day of the attack of , she was ordered to stay away from the area and returned to Dundee on 3 October. Between 3 October 1939 and 13 March 1940, she refitted in Dundee, then conducted training exercises. On 22 March, she left port for another patrol, in the western approach to the Skagerrak strait. Swordfish sighted only neutral ships during this patrol and returned to Blyth on 8 April. Swordfish departed Blyth again on 16 April for another patrol in the Skagerrak. On 20 April, she sighted and attacked a German convoy, made up of Three heavy transports and four escorts in position . Her torpedoes missed, but Swordfish was attacked with depth charges until the evening by the convoy's escorts. Two days later, she sighted another convoy of trawlers and escorts but did not attack them due to their shallow draught, which would have caused the torpedoes to run under the ships. In the morning of 26 April, Swordfish sighted several drifting mines coming towards her; one of these struck as Swordfish was diving, but failed to explode. Swordfish returned to Blyth two days later, ending her fourth patrol. Swordfish conducted an uneventful patrol from 10 to 25 May in the North Sea. On 5 June, she departed Blyth for her sixth war patrol in the North Sea. The next day, she was bombed in error by a Lockheed Hudson of No. 224 Squadron RAF, but no damage was sustained. After returning to Blyth, Swordfish conducted another patrol, but only aircraft were sighted. On her eighth patrol, she same across the yacht Maski, which was fleeing to the United Kingdom. After taking in the yacht's crew, Swordfish sank it with gunfire. She then conducted a tenth and eleventh patrol, missing several ships with torpedoes. Nothing was heard from her following her departure from