Marines, also known as a Marine Corps or naval infantry, are typically an infantry force that specializes in the support of naval and army operations at sea and on land, as well as the execution of their own operations. In the majority of countries, the marine force is an integral part of the navy, but it can also be under the army like the Troupes de marine (French Marines) and Givati Brigade (Israeli Marines), or form an independent service branch like the United States Marine Corps and the Royal Marines (although the United States Marine Corps is under the United States Department of the Navy, and the Royal Marines come under Her Majesty's Naval Service).

In the earliest day of naval warfare, there was little distinction between sailors and soldiers on a warship. The oarsmen of Ancient Greek and 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.