A galley is a type of ship that is propelled mainly by oars. The galley is characterized by its long, slender hull, shallow draft, and low freeboard (clearance between sea and gunwale). Virtually all types of galleys had sails that could be used in favorable winds, but human effort was always the primary method of propulsion. This allowed galleys to navigate independently of winds and currents. The galley originated among the seafaring civilizations around the Mediterranean Sea in the late second millennium BC and remained in use in various forms until the early 19th century in warfare, trade, and piracy.

Galleys were the warships used by the early Mediterranean naval powers, including the Greeks, Illyrians, Phoenicians, and Romans. They remained the dominant types of vessels used for war and piracy in the Mediterranean Sea until the last decades of the 16th century. As warships, galleys carried various types of weapons throughout their long existence, including rams, catapults, and cannons, but also relied on their large crews to overpower enemy vessels in boarding actions. They were the first ships to effectively use heavy cannons as anti-ship weapons. As highly efficient gun platforms, they forced changes in the design of medieval seaside fortresses as well as refinement of sailing warships.

Galleys were the most common warships in the Atlantic Ocean during the Middle Ages, and later saw limited use in the Caribbean, the Philippines, and the Indian Ocean in the early modern period, mostly as patrol craft to combat pirates. From the mid-16th century galleys were in intermittent use in the Baltic Sea, with its short distances and extensive archipelagoes. The zenith of galley usage in warfare came in the late 16th century with battles like that at Lepanto in 1571, one of the largest naval battles ever fought. By the 17th century, however, sailing ships and hybrid ships like the xebec displaced galleys in naval warfare. There was a minor revival of galley warfare in the 18th century in the wars among Russia, Sweden, and Denmark.