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 . She also damaged six merchant ships of , two auxiliary warships of and one warship of 1,020 tons (the US Navy destroyer before fleeing the fight). She went missing some time after March 23, 1918.
U-61 was originally thought to have been sunk in a depth charge attack by PC51 on 26 March 1918. This attack was actually against , which was able to continue on patrol despite the attack.