O 21, laid down K XXI was an O 21-class submarine of the Royal Netherlands Navy that saw service during World War II.

The submarine was laid down on 20 November 1937 as K XXI at the Koninklijke Maatschappij De Schelde, Vlissingen During construction she was renamed O 21, and was finally launched on 21 October 1939.

[citation needed] Following the German invasion of 10 May 1940, the O 21 was hastily commissioned, still incomplete, and sailed for England on 12 May together with her sister O 22 and the tugboat B.V. 37, to be finally completed at the Navy yard in Rosyth.

[3] During the war she operated around England, the Mediterranean Sea, Colombo in the Indian Ocean and Fremantle off the west coast of Australia.

[4] She survived the war and was decommissioned on 2 November 1957 and sold for scrap the following year to the firm v. Beekum in Alkmaar.