SM U-61 was a German Type U 57 U-boat commissioned and deployed to operate off the coast of the British Isles and attack coastal shipping as part of the U-boat Campaign during World War I. In a 15-month career spanning nine war patrols, U-61 plagued allied shipping in the Atlantic Ocean during the German war on Allied trade (Handelskrieg). She sank 33 Allied ships, totalling 84,564 gross register tons (GRT). She also damaged six merchant ships of 21,054 GRT, two auxiliary warships of 3,424 GRT and one warship of 1,020 tons (the US Navy destroyer USS Cassin before fleeing the fight). She went missing some time after March 23, 1918.