A prison ship, 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. Some prison ships were hulked. 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.

The terminology "hulk" comes from the Royal Navy meaning a ship incapable of full service either through damage or from initial non-completion. In England in 1776, during the reign of King George III, due to a shortage of prison space in London, the concept of "prison hulks" moored in the Thames, was introduced to meet the need for prison space. The first such ship came into use on 15 July 1776 under command of Mr Duncan Campbell and was moored at Barking Creek with prisoners doing hard labour on the shore during daylight hours.

The vessels were a common form of internment in Britain and elsewhere in the 18th and 19th centuries. Charles F. Campbell writes that around 40 ships of the Royal Navy were converted for use as prison hulks. Other hulks included HMS Warrior, which became a prison ship at Woolwich in February 1840. One was established at Gibraltar, others at Bermuda (the Dromedary), at Antigua, off Brooklyn in Wallabout Bay, and at Sheerness. Other hulks were anchored off Woolwich, Portsmouth, Chatham, Deptford, and Plymouth-Dock/Devonport. HMS Argenta, originally a cargo ship with no portholes, was acquired and pressed into service in Belfast Lough Northern Ireland to enforce the Civil Authorities (Special Powers) Act (Northern Ireland) 1922 during the period around the Irish Catholics' Bloody Sunday (1920). Private companies owned and operated some of the British hulks holding prisoners bound for penal transportation to Australia and America.

HMP Weare was used by the British as a prison ship between 1997 and 2006. It was towed across the Atlantic from the United States in 1997 to be converted into a jail. It was berthed in Portland Harbour in Dorset, England.

During the American War of Independence, the British used a system of prison ships to imprison American prisoners of war. Many of these prison ships were moored in Wallabout Bay near New York City, which was a major British stronghold during the conflict. Conditions onboard these ships were abysmal due to overcrowding, the poor quality of the ships, mistreatment from guards and contaminated water and food. Waves of disease frequently spread through the ships, which combined with starvation killed 12,000 American prisoners of war. The bodies of those who died were mostly hastily buried along the shore, and were commemorated by the Prison Ship Martyrs' Monument in Fort Greene Park, Brooklyn.

Christopher Vail, of Southold, who was aboard one such prison ship, HMS Jersey in 1781, later wrote:

When a man died he was carried up on the forecastle and laid there until the next morning at 8 o'clock when they were all lowered down the ship sides by a rope round them in the same manner as tho' they were beasts. There was 8 died of a day while I was there. They were carried on shore in heaps and hove out the boat on the wharf, then taken across a hand barrow, carried to the edge of the bank, where a hole was dug 1 or 2 feet deep and all hove in together.

In 1778, Robert Sheffield, of Stonington, Connecticut, escaped from a British prison ship and told his story in the Connecticut Gazette, printed July 10, 1778. He was one of 350 prisoners held in a compartment below the decks.