A fire ship is a ship that is filled with explosives, deliberately set on fire and steered (or, if possible, allowed to drift) into an enemy fleet in order to destroy ships or create panic and make them break formation.

Warships of the age of sail were highly vulnerable to fire. With seams caulked with tar, ropes greased with fat, and holds full of gunpowder, there was little that would not burn. Accidental fires destroyed many ships, so fire ships presented a terrifying threat.

With the wind in exactly the right direction a fire ship could be cast loose and allowed to drift onto its target, but in most battles fire ships were equipped with skeleton crews to steer the fire ship onto the target (the crew were expected to abandon ship at the last moment and escape in the ship's boat). Fire ships were most devastating against ships at anchor or otherwise restricted in movement. At sea, a well-handled ship could evade a fire ship and disable it with cannon fire. Ships used as fire ships were old or inexpensive vessels.

Notable fire ship attacks include:

• Francis Drake's attack on the Spanish Armada moored at Gravelines in 1588. The fire ships did no damage, but the Spanish scattered in panic and were easy prey for English ships.
• Maarten Tromp's attack on the Spanish fleet moored off the Kent coast in the battle of the Downs in 1639. The Spanish fleet was destroyed.
• Michiel de Ruyter's attack on the anchored English fleet at the battle of Solebay in 1672 in which HMS Royal James was burned and her captain Edward Montagu killed.
• The Russian attack on the Turkish fleet at the battle of Chesma, 1770.
• Thomas Cochrane's attack on the French in the battle of the Basque Roads in 1809.
• Many Greek attacks on Turkish ships during the Greek War of Independence, 1821–1832

Use of fire ships was discontinued after the end of wooden fighting ships. An extension of the concept was however used in Operation Chariot of World War II, in which the old destroyer HMS Campbeltown was packed with explosives and rammed into the dry dock at Saint-Nazaire, France.