O 23, laid down as K XXIII, was an O 21-class submarine of the Royal Netherlands Navy that saw service during World War II.

The submarine was ordered on 9 June 1937 and laid down on 12 October 1937 as K XXIII at the Rotterdamsche Droogdok Maatschappij, Rotterdam.

Following the German invasion of 10 May 1940, O 23 was hastily commissioned, still incomplete, and sailed for England on 13 May to be completed at the Thornycroft shipyard at Southampton.

O 23 made twenty patrols during the war in the course of which she sank or damaged five ships.

She survived the war and was decommissioned on 1 December 1948, being sold for scrap in April of the following year.