HMS H1 was a British H-class submarine built by Canadian Vickers Co., Montreal. She was laid down on 11 January 1915 and was commissioned on 26 May 1915. H1 crossed the Atlantic from St John's, Newfoundland to Gibraltar escorted by the armed merchant cruiser HMS Calgarian. She was accompanied by H2, H3 and H4. H1 mistakenly sank Italian submarineH5 off Cattaro on 15 April 1918. H1 was sold on 7 March 1921 in Malta.