German submarine U-45 was a Type VIIB U-boat of Nazi Germany's Kriegsmarine during World War II. She was ordered on 21 November 1936 and laid down on 23 February 1937 at Germaniawerft in Kiel as yard number 580. She was launched on 27 April 1938 and commissioned on 25 June 1938 under the command of Kapitänleutnant (Kptlt.) Alexander Gelhaar.