German submarine U-45 was a Type VIIB U-boat of Nazi Germany's Kriegsmarine during World War II.

The submarine was powered by two Germaniawerft F46 four-stroke, six-cylinder supercharged diesel engines producing a total of 2,800 to 3,200 metric horsepower (2,060 to 2,350 kW; 2,760 to 3,160 shp) for use while surfaced, two BBC GG UB 720/8 double-acting electric motors producing a total of 750 metric horsepower (550 kW; 740 shp) for use while submerged.

[1] During her Kriegsmarine service, U-45 conducted only two war patrols and sank two vessels for a loss of 19,313 gross register tons (GRT).

While operating with others in an attack on an Allied convoy, U-45 was sunk by depth charges from the British destroyers HMS Inglefield, Ivanhoe and Intrepid on 14 October 1939 southwest of Ireland.

U-45 also attacked but failed to damage the 10,350 ton British steam merchantman Karamea; the single torpedo fired at this ship detonated prematurely (a common problem early in the war).