HMS Forth was a submarine depot ship adapted to operate and maintain the Royal Navy's nuclear-powered submarines.

She was commissioned shortly before WWII, on 14 May 1939. and served at Holy Loch on the Clyde for a period. Later in the war Forth was stationed at Trincomalee.

During her stay in Malta in the 1950s she was moored on the east side of Msida Creek.  She left Malta in 1960.

HMS Forth took the first Hovercraft to Australia in 1968

On 5 February 1972 she was renamed HMS Defiance

She was decommissioned on 1 January 1979 and sold for scrapping on 25 July 1985