Piracy is an act of robbery or criminal violence by ship or boat-borne attackers upon another ship or a coastal area, typically with the goal of stealing cargo and valuable goods, or taking hostages. Those who conduct acts of piracy are called pirates, and vessels used for piracy are called pirate ships. The earliest documented instances of piracy were in the 14th century BC, when the Sea Peoples, a group of ocean raiders, attacked the ships of the Aegean and Mediterranean civilisations. Narrow channels which funnel shipping into predictable routes have long created opportunities for piracy, as well as for privateering and commerce raiding.

Historic examples of such areas include the waters of Gibraltar, the Strait of Malacca, Madagascar, the Gulf of Aden, and the English Channel, whose geographic structures facilitated pirate attacks. The term piracy generally refers to maritime piracy, although the term has been generalized to refer to acts committed on land, in the air, on computer networks, and (in science fiction) outer space. Piracy usually excludes crimes committed by the perpetrator on their own vessel (e.g. theft), as well as privateering, which implies authorization by a state government.

Piracy or pirating is the name of a specific crime under customary international law and also the name of a number of crimes under the municipal law of a number of states. In the 21st century, seaborne piracy against transport vessels remains a significant issue, with estimated worldwide losses of US$25 billion in 2023, increased from US$16 billion in 2004.

The waters between the Red Sea and the Indian Ocean, off the Somali coast and in the Strait of Malacca and Singapore have frequently been targeted by modern pirates armed with automatic firearms and occasionally explosive weaponry. They often use small motorboats to attack and board ships, a tactic that takes advantage of the small number of crew members on modern cargo vessels and transport ships. The international community is facing many challenges in bringing modern pirates to justice, as these attacks often occur in international waters. Nations have used their naval forces to repel and pursue pirates, and some private vessels use armed security guards, high-pressure water cannons, or sound cannons to repel boarders, and use radar to avoid potential threats.

Romanticised accounts of piracy during the Age of Sail have long been a part of Western pop culture. The two-volume A General History of the Pyrates, published in London in 1724, is generally credited with bringing key piratical figures and a semi-accurate description of their milieu in the "Golden Age of Piracy" to the public's imagination. The General History inspired and informed many later fictional depictions of piracy, most notably the novels Treasure Island (1883) and Peter Pan (1911), both of which have been adapted and readapted for stage, film, television, and other media across over a century. More recently, pirates of the "golden age" were further stereotyped and popularized by the Pirates of the Caribbean film franchise, which began in 2003.

The English word "pirate" is derived from the Latin pirata ("pirate, corsair, sea robber"), which comes from Greek πειρατής (peiratēs), "brigand", from πειράομαι (peiráomai), "I attempt", from πεῖρα (peîra), "attempt, experience". The meaning of the Greek word peiratēs literally is "anyone who attempts something". Over time it came to be used of anyone who engaged in robbery or brigandry on land or sea. The term first appeared in English c. 1300. Spelling did not become standardised until the eighteenth century, and spellings such as "pirrot", "pyrate" and "pyrat" occurred until this period.

The earliest documented instances of piracy are the exploits of the Sea Peoples who threatened the ships sailing in the Aegean and Mediterranean waters in the 14th century BC. In classical antiquity, the Phoenicians, Illyrians and Tyrrhenians were known as pirates. In the pre-classical era, the ancient Greeks condoned piracy as a viable profession; it apparently was widespread and "regarded as an entirely honourable way of making a living". References are made to its perfectly normal occurrence in many texts including in Homer's Iliad and Odyssey, and abduction of women and children to be sold into slavery was common. By the era of Classical Greece, piracy was looked upon as a "disgrace" to have as a profession.

In the 3rd century BC, pirate attacks on Olympus in Lycia brought impoverishment. Among some of the most famous ancient pirateering peoples were the Illyrians, a people populating the western Balkan peninsula. Constantly raiding the Adriatic Sea, the Illyrians caused many conflicts with the Roman Republic. It was not until 229 BC when the Romans decisively beat the Illyrian fleets that their threat was ended. During the 1st century BC, there were pirate states along the Anatolian coast, threatening the commerce of the Roman Empire in the eastern Mediterranean. On one voyage across the Aegean Sea in 75 BC, Julius Caesar was kidnapped and briefly held by Cilician pirates and held prisoner in the Dodecanese islet of Pharmacusa. The Senate invested the general Gnaeus Pompeius Magnus with powers to deal with piracy in 67 BC (the Lex Gabinia), and Pompey, after three months of naval warfare, managed to suppress the threat.