Gaff rig is a sailing rig in which the mainsail is a four-cornered fore-and-aft rigged sail controlled at its head by a spar called the gaff.

A sail hoisted from a gaff is called a gaff rigged sail. Any mast may carry a gaff rigged course sail.

Gaff rig remains the most popular rig for schooner and barquentine mainsails and other course sails, and spanker sails on a square rigged vessel are always gaff rigged. On other rigs, particularly the sloop, ketch and yawl, gaff rigged sails were once common but have now been largely replaced by the bermuda sail.

On larger gaff rigged vessels, the gaff is hoisted by two halyards:

• The throat or main halyard lifts the end closer to the mast, and bears the main weight of the sail and the tension of the luff.
• The peak halyard lifts the end further from the mast, and bears the leach tension.

On such rigs, a triangular fore-and-aft topsail called a gaff sail may be carried between the gaff and the upper mast.

On smaller vessels, the gaff is raised by a single halyard running on a wire gunter. On these rigs the gaff may be very nearly vertical, and a topsail is never carried.