Privateer, a privately owned armed vessel commissioned in former times by a warring nation to seize enemy ships. The officers and men of such a ship were also called privateers. Privateering enriched many captains and crews, since they received all or most of the ships and cargoes seized. Many privateers turned pirate by continuing their attacks after their commissions, called letters of marque, became invalid because the war for which they were issued had ended. Buccaneers, who preyed on Spanish fleets sailing from America, were usually privateers with secret commissions issued by enemies of Spain.

Privateering was engaged in from the 13th century into the 19th. During the age of exploration and colonization, it was a general practice among the great nations of Europe, providing them with a convenient way of maintaining navies adequate for their incessant warfare. During the American Revolution, the United States had scarcely any navy, but had more than 1,000 ships commissioned as privateers.

By the Declaration of Paris (1856) most maritime nations, but not the United States, agreed to abolish privateers. The Union did not employ privateers in the Civil War, but the Confederacy commissioned a few. Thereafter privateers went out of existence.