German submarine U-21 was a Type IIB U-boat of the Nazi German Kriegsmarine. Her keel was laid down 4 March 1936, by Germaniawerft of Kiel as 'werk' 551. She was commissioned on 3 August 1936. During World War II, she conducted operations against enemy shipping. U-21 went on seven war patrols, sinking five ships, one auxiliary warship and damaging one warship.