HMS R12 was a British R class submarine built by Cammell Laird at Birkenhead. She was laid down on 1 December 1917 and launched on 9 April 1918. R12 was commissioned on 29 October 1919. She came too late to see any combat in World War I, like most of the other R class submarines. R12 was paid off in December 1919, and sold on 21 February 1923 to J. Smith.