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.

A hulk is a ship that is afloat, but incapable of going to sea. Hulk may be used to describe a ship that has been launched but not completed, an abandoned wreck or shell, or to refer to a ship whose propulsion system is no longer maintained, or has been removed altogether. The word hulk also may be used as a verb: a ship is "hulked" to convert it to a hulk. The verb was also applied to crews of Royal Navy ships in dock, who were sent to the receiving ship for accommodation, or "hulked". Hulks have a variety of uses such as housing, prisons, salvage pontoons, gambling sites, naval training, or cargo storage.