HMS Inconstant was an iron screw frigate of the Royal Navy. She was launched on 12 November 1868 and became a training ship in 1906, renamed Impregnable II. She became the Navy's torpedo school ship in January 1922 and was renamed Defiance IV, and Defiance II in December 1930, before being finally scrapped in 1956. She was the first of three steam-assisted but fully masted frigates with iron hulls, sheathed with a double layer of wood, that were built for the Royal Navy.