Square rig is a generic type of sail and rigging arrangement in which a sailing vessel's primary driving sails are carried on horizontal spars that are perpendicular (or square) to the median plane of the keel and masts of the vessel. These spars are called yards and their tips, outside the lifts, are called the yardarms. A ship mainly rigged so is called a square-rigger.

In "Jackspeak" (Royal Navy slang), it also refers to the dress uniform of Junior Ratings.

Single sail square rigs were used by the ancient Egyptians, the Phoenicians, the Greeks, the Romans, and the Celts. Later the Scandinavians, the Germanic peoples, and the Slavs adopted the single square-rigged sail, with it becoming one of the defining characteristics of the classic “Viking” ships.