HMS L12 was a British L class submarine built by Vickers, Barrow-in-Furness. She was laid down on 22 January 1917 and was commissioned on 30 June 1918. HMS L12 was converted into a minelayer along with HMS L11. She and L11 were the only group 1 boats to be converted into minelayers. HMS L12 collided with HMS H47 off Milford Haven, Wales on 9 July 1929. Luckily the boat resurfaced and returned to Milford Haven. Unfortunately, three sailors died. HMS L12 was sold on 16 February 1932 in Newport.