HMS H21 was a British H class submarine built by Vickers Limited, Barrow-in-Furness, as the first of the Batch 3 H class submarines. She was launched on 20 October 1917 and was commissioned on 28 January 1918. H21 was an improved design of the H class which produced a larger displacement and the latest 21 inch torpedo tubes replacing the old 18 inch torpedo tubes. This would be incorporated into all batch 3 H class submarines. HMS H21 was sold to John Cashmore Ltd on 13 July 1926 for scrapping at Newport.