HMS Maidstone was a submarine depot ship of the Royal Navy. It operated in the Mediterranean Sea, Indian Ocean and Pacific Ocean during the Second World War. It was later used as a barracks ship and then a prison ship in Northern Ireland.

A prison ship, often more accurately described as a prison hulk, is a current or former seagoing vessel that has been modified to become a place of substantive detention for convicts, prisoners of war or civilian internees. While many nations have deployed prison ships over time, the practice was most widespread in 18th- and 19th-century Britain, as the government sought to address the issues of overcrowded civilian jails on land and an influx of enemy detainees from the War of Jenkins' Ear, the Seven Years' War and the French Revolutionary and Napoleonic Wars.