A studding sail or studsail is a sail used to increase the sail area of a square rigged vessel in light winds.

It is an extra sail hoisted alongside a square-rigged sail on an extension of its yardarm. It is named by prefixing the word studding to the name of the working sail alongside which it is set.

Studding sails have also been used to increase the sail area of a fore-and-aft spanker, again by extending the upper spar.