A sloop is a single-masted boat. Unlike cutters and other boats with only one mast, a sloop's mast is located further forward in a boat and supports a smaller forward sail. A sloop generally carries a headsail and a mainsail, but the term can refer to a number of different craft, from a 'Bermuda sloop' (a modern yachting sloop) to a 'sloop-of-war' (a large craft historically used by the British Navy).

The sloop design is ideal for upwind sail, and is considered a good general design for most sailing purposes. As a light rig with minimal lines and spars, a sloop has less drag on the sails to deal with. The same characteristic also makes for flatter sails, and less sideways force on the sails. Sideways force is an important concern for preventing heeling in a sailing craft — keeping the craft upright and preventing the weight of the sails from tipping it over. A sloop relies on its keel and flat sails to stay upright.

Bermuda sloops are the most common type of sloop found in modern sailing. They are popular with yachters and for racing, because of the sloop's ability for upwind sailing. Bermuda rigs are especially known for their speed and maneuverability. Their design usually incorporates a bowspirit (a spar extending from the bow of the craft) in order to increase the amount of sail the sloop carries. The Bermuda design dates from the 17th century and has been in regular use for four centuries.

The typical racing sloop's design is based on the Bermuda rig, but focuses on improving speed and movement to the exclusion of all else. Such sloops are not necessarily seaworthy, and run high risk of capsizing. Most racing competitions, therefore, have established firm rules regarding the reliability of competing craft, as well as limiting new technology.