A mainsail is a sail rigged on the main mast of a sailing vessel.

• On a square rigged vessel, it is the lowest and largest sail on the main mast.

• On a fore-and-aft rigged vessel, it is the sail rigged aft of the main mast. The sail's foot is normally attached to a boom. (In extremely heavy weather, the mainsail may be lowered, and a much smaller trysail hoisted in its place).

Historical fore-and-aft rigs used a four-sided gaff rigged mainsail, sometimes setting a gaff topsail above it.

Whereas once the mainsail was typically the largest sail, today the mainsail may be smaller than the jib or genoa; Prout catamarans typically have a mainmast stepped further aft than in a standard sloop, so that the mainsail is much smaller than the foresail.