Marines (or naval infantry) are military personnel generally trained to operate on both land and sea, with a particular focus on amphibious warfare. Historically, the main tasks undertaken by marines have included raiding ashore (often in support of naval objectives) and the boarding of vessels during ship-to-ship combat or capture of prize ships. Marines also assisted in maintaining security, discipline, and order aboard ships (reflecting the historically pressed-nature of the rest of the ship's company and the risk of mutiny). While maintaining many of their historical roles, in modern times, marines also engage in duties including rapid-response operations, humanitarian aid, disaster relief, special operations roles, and counter-terrorism operations. In most nations, marines are an integral part of that state's navy, such as the United Kingdom's Royal Marines or Russia's Naval Infantry; in some countries their marine forces can instead be part of the land army, such as the French Troupes de Marine, or, more uncommonly, a nation’s marine forces may be an independent military branch such as the United States Marine Corps or the Ukrainian Marine Corps.

The exact term "marine" is not found in many languages other than English. In French-speaking countries, two terms which could be translated as "marine", but do not translate exactly: troupes de marine (marine troops) and fusiliers-marins (marine riflemen) and fuzileiros navais in Portuguese (lit. 'Naval fusiliers'). The word marine means "navy" in many European languages such as Dutch, French, German, Italian and Norwegian. “Naval infantry” may also refer to sailors forming both temporary and permanent infantry units, such as the British WWI-era 63rd (Royal Naval) Division (an infantry division made-up of Royal Navy sailors and Royal Marines on a semi-permanent basis) or the Imperial Japanese Naval Landing Forces (ad-hoc formations of Imperial Japanese Navy sailors temporarily pressed into service as infantry).

In the early days of naval warfare, there was little distinction between sailors and soldiers on a warship. The oarsmen of Ancient Greek and Ancient Roman ships had to be capable of fighting the rowers of opposing ships hand-to-hand, though hoplites began appearing on Greek ships specifically for the boarding of enemy ships.

The Roman Republic was the first to understand the importance of professional soldiers dedicated to melee combat onboard of ships. During the First Punic War, Roman crews remained inferior in naval experience to the Carthaginians and could not hope to match them in naval tactics, which required great fleet maneuverability and tactical experience. The Romans therefore employed a novel weapon which changed sea warfare to their advantage — they equipped their ships with the corvus, a long pivoting plank with a beak-like spike on the underside for hooking onto enemy ships, possibly developed earlier by the Syracusans against the Athenians during the Sicilian Expedition of the Peloponnesian War. Using it as a boarding bridge, Roman infantrymen were able to invade an enemy ship, transforming sea combat into a version of land combat, where the Roman legionaries had the upper hand. During the early Principate, a ship's crew, regardless of its size, was organized as a centuria. Crewmen could sign on as naval infantry (called Marinus), rowers/seamen, craftsmen and various other jobs, though all personnel serving in the imperial fleet were classed as milites ("soldiers"), regardless of their function; only when differentiation with the army was required, were the adjectives classiarius or classicus added. The Roman Navy's two fleet legions, I Adiutrix and II Adiutrix, were among the first distinct naval infantry units.

The first organized marine corps was created in Venice by the Doge Enrico Dandolo when he created the first regiment of ten companies spread on several ships. That corps participated in the conquest of Byzantium (1203–1204), later officially called "Fanti da Mar" (sea infantry) in 1550., Venice also had dedicated naval expeditionary corps of naval infantry recruited primarily from Dalmatia from the local population called the Oltremarini (overseas troops)

Later, Spanish King Carlos I assigned the naval infantry of the Compañías Viejas del Mar de Nápoles (Naples Sea Old Companies) to the Escuadras de Galeras del Mediterráneo (Mediterranean Galley Squadrons) in 1537, progenitors of the current Spanish Navy Marines (Infantería de Marina) corps, making them the oldest marine corps still in active service in the world.

The English noun marine is from the adjective marine, meaning "of the sea", via French marin ("of the sea") from Latin marinus ("of the sea") itself from mare ("sea"), from Proto-Indo-European *móri ("body of water, lake") (cognate with Old English mere ("sea, lake"), Dutch meer, German Meer, all from Proto-Germanic *mari).

The word marine was originally used for the marine-type forces of England; however, the word marine or marina means "navy" in many European languages, including Dutch, French, Italian, German, Spanish, Danish, and Norwegian. Because of this, exact one-word translations for the English term "marines" do not exist in many other languages (with the notable exception of the Dutch word marinier). This can lead to misunderstandings when translating. Marine forces in non-English speaking countries typically have names that translate in English to naval infantry or coastal infantry. In French-speaking countries, two phrases exist which could be translated as marine, "troupes de marine" and "fusiliers-marins"; similar phrases exist elsewhere, e.g., in Portuguese Fuzileiros Navais.