HMS H21 was a British H-class submarine built by Vickers Limited, Barrow-in-Furness, as the first of the Batch 3 subgroup. 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.
Like all post-H20 British H-class submarines, H21 had a displacement of at the surface and while submerged. It had a total length of , a beam of , and a draught of . It contained a diesel engines providing a total power of and two electric motors each providing power. The use of its electric motors made the submarine travel at . It would normally carry of fuel and had a maximum capacity of .
The submarine had a maximum surface speed of and a submerged speed of . Post-H20 British H-class submarines had ranges of at speeds of when surfaced. H21 was fitted with an anti-aircraft gun and four torpedo tubes. Its torpedo tubes were fitted to the bow and the submarine was loaded with eight torpedoes. It is a Holland 602 type submarine but was designed to meet Royal Navy specifications. Its complement was twenty-two crew members.