A fire ship or fireship is a large wooden vessel set on fire to be used against enemy ships during a ramming attack or similar maneuver. Fireships were used to great effect against wooden ships throughout naval military history up until the advent of metal-hulled ships; they could also serve a considerable function in shock and awe strategies to harm the morale of enemy crews. Ships used for fireship tactics were typically purpose-built or retrofitted from surplus or obsolete vessels, either of which could be filled with gunpowder or other combustibles before a battle, but could also be improvised from warships in active combat purposely set on fire during engagements, such as if a vessel expended its munitions or had some other reason to be abandoned in battle.

Specialized fire ships included the massive Dutch hellburners, also called "explosion ships", which were fitted with large explosive stores designed to detonate upon contact with the enemy. Fireships were used to great effect by the outgunned English fleet against the Spanish Armada during the Battle of Gravelines, the Dutch in the raid on the Medway, Chinese warlord Sun Quan in the Battle of Red Cliffs, and the Greeks in their war of independence.

The Tyrians launched a fire ship that destroyed two Macedonian siege towers and other siege engines at the Siege of Tyre (332 BC).

During the Third Punic War, Carthaginian fire ships destroyed many Roman vessels and came close to burning the entire Roman fleet at the Battle of Lake Tunis in 149 BC.

During the end of the Han dynasty, at the Battle of Red Cliffs (208 CE) on the Yangtze River, Huang Gai assaulted Cao Cao's naval forces with a fire ship filled with bundles of kindling, dry reeds, and fatty oil.

Fire ships were employed to decisive effect by the Vandals against the armada sent by the Eastern Roman Empire, in the Battle of Cape Bon (468).

The invention of Greek fire in 673 increased the use of fire ships, at first by the Greeks and afterward by other nations as they came into possession of the secret of manufacturing this substance. In 951 and again in 953 Russian fleets narrowly escaped destruction by fire ships.^{[citation needed]}

While fire ships were used in the Medieval period, notably during the Crusades, these were typically ships that were set up with combustibles on an ad hoc basis. The career of the modern fire ship, as a naval vessel type designed for this particular function and made a permanent addition to a fleet, roughly parallels the era of cannon-armed sailing ships, beginning with the defeat of the Spanish Armada in 1588 and lasting until the Allied victory over the Turks at the Battle of Navarino in 1827. The first modern fireships were put to use in early 17th century Dutch and Spanish fleet actions during the Thirty Years War. Their use increased throughout that century, with purpose-built fireships a permanent part of many naval fleets, ready to be deployed whenever necessary. Initially small and often obsolete smaller warships were chosen as fireships but by 1700 fireships were being purpose-built with specific features for their role. Most were adaptations of the usual small warships of the day – brigs or ship-rigged sloops-of-war with between 10 and 16 guns. The practical design features of purpose-built fireships included a lattice-work false deck below the planks of the main deck – the planks would be removed and the combustibles and explosives stacked on the lattice, which gave good draught and ensured the fire would hold and spread. A number of square-section chimneys would be let into the forecastle and quarterdeck to also help ensure a good draught for the fire. The gunports would be hinged at the bottom (rather than the top as on other warships) so that they would be kept open by gravity rather than ropes (which would otherwise burn thorough), further ensuring a good air supply. On the other hand, the lower parts of the masts would be surrounded by 'coffer dams' to ensure that the fire would not bring down the masts prematurely and thus deprive the fireship of motive power. Grappling hooks would be fitted to the ends of the yardarms so that the fireship would become entangled in its target's rigging. A large sally-port door was let into the rear quarter of the ship (usually the starboard side) to allow easy exit for the crew once the fire had been set and lit. There was often a chain fixed here for mooring the escape boat rather than a rope that may have been damaged by the fire. Because fireships were used relatively rarely and only in specific tactical conditions even in their heyday, and there was always demand for small cruisers and warships, most purpose-built 'fireships' served long careers as ordinary warships without ever being used for their actual purpose. Of the five fireships used in Holmes's Bonfire of 1666 three had been in service with the Royal Navy for over a decade before being deployed on their final mission.