A Prison Ship is a boat or ship that is used as a prison. They are considered an antiquated method of housing prisoners and have largely been replaced by land based prisons. The United Kingdom Government established in 1997 a new prison ship, HMP Weare as a temporary measure to ease prison overcrowding. HMP Weare was docked at the disused Royal Navy dockyard at Portland, Dorset. On 09 March 2005 it was announced that HMP Weare was to close.

The HMS Maidstone was used as a prision ship in Northern Ireland in the 1970's for suspected Nationalist terrorists held without trial. Sinn Fein president Gerry Adams spent time on the Maidstone in 1972, before being released in order to take part in peace talks.