A letter of marque and reprisal was a government license in the Age of Sail that authorized a private person, known as a privateer or corsair, to attack and capture vessels of a foreign state at war with the issuer, licensing international military operations against a specified enemy as reprisal for a previous attack or injury. Captured naval prizes were judged before the government's admiralty court for condemnation and transfer of ownership to the privateer.

A common practice among Europeans from the late Middle Ages to the 19th century, cruising for enemy prizes with a letter of marque was considered an honorable calling that combined patriotism and profit. Such legally authorized privateering contrasted with unlicensed captures of random ships, known as piracy, which was universally condemned. In practice, the differences between privateers and pirates were sometimes slight, even merely a matter of interpretation.

The terms "letter of marque" and "privateer" were sometimes used to describe the ships which typically operated under the marque-and-reprisal licences. In this context, a letter of marque was a lumbering, square-rigged cargo carrier that might pick up a prize if the opportunity arose in its normal commerce. In contrast, the term privateer generally referred to a fighting vessel, fore-and-aft rigged, fast, and weatherly.

Letters of marque allowed governments to fight their wars using mercenary private captains and sailors in place of their own navies as a measure to save time and money. Instead of building, funding, and maintaining a navy in times of peace, governments would wait until the start of a war to issue letters of marque to privateers, who financed their own ships in expectation of prize money.

Marque derives from the Old English mearc, which is from the Germanic *mark-, which means boundary, or boundary marker. This is derived from the Proto-Indo-European root *merǵ-, meaning boundary, or border. The French marque is from the Provençal language marca, which is from marcar, also Provençal, meaning to seize as a pledge.

According to the Oxford English Dictionary, the first recorded use of "letters of marque and reprisal" was in an English statute in 1354 during the reign of King Edward III. The phrase referred to "a licen[c]e granted by a sovereign to a subject, authorizing him to make reprisals on the subjects of a hostile state for injuries alleged to have been done to him by the enemy's army".

During the Middle Ages, armed private vessels enjoying their sovereign's tacit consent, if not always an explicit formal commission, regularly raided shipping of other states, as in the case of the English Sir Francis Drake's attacks on Spanish shipping. Queen Elizabeth I (despite protestations of innocence) took a share of the prizes. Dutch jurist Hugo Grotius's 1604 seminal work on international law, De Iure Praedae (Of The Law of Prize and Booty), was an advocate's brief defending Dutch raids on Spanish and Portuguese shipping.

King Henry III of England first issued what later became known as privateering commissions in 1243. These early licences were granted to specific individuals to seize the King's enemies at sea in return for splitting the proceeds between the privateers and the Crown.