SM U-45[Note 1] was one of the 329 submarines serving in the Imperial German Navy in World War I. U-45 was engaged in the naval warfare and took part in the First Battle of the Atlantic.

U-45 was torpedoed and sunk in the Atlantic Ocean northwest of the Shetland Islands (55°48′N 7°30′W / 55.800°N 7.500°W / 55.800; -7.500) by the Royal Navy submarine HMS D7 on 12 September 1917 with the loss of 43 of her 45 crewmen.