A gag is a device worn in or on the mouth to prevent the wearer from speaking. Gags are commonly used in conjunction with bondage during BDSM play and are usually worn by submissives to give the dominant partner more control. Gags can be used to make a submissive more helpless and as a form of punishment or humiliation.

Ball gags are perhaps one of the most popular types of gags used during BDSM play. These gags have soft, solid rubber balls that are inserted into the mouth with straps to secure them to the head. Wiffle ball gags are very similar, but they have small plastic balls with several holes in them making it easier for the wearer to breath. Open-mouth gags are designed to keep the mouth open so the wearer can perform oral sex. Bit gags have rubber bits that are inserted into the mouth for pony play. Other gags include penis gags, forniphilic gags, and cleave gags.

Safety is a big issue when using a gag during BDSM play because there is a danger of asphyxiation when a person's airways are obstructed. Always make sure that a gag fits the wearer and is not too tight. Never leave a gagged submissive alone. Also, never obstruct the wearer's nose when using a gag. Never use a gag on a person who is ill or is suffering from allergies.

Because a person is prevented from speaking when using a gag, he or she will be unable to utter a safeword during a scene. An alternative to the safeword should be decided on beforehand in this case. A hand signal, such as dropping a handkerchief or clenching a fist, can be used, for instance, as can eye signals, such as a series of blinks. Many BDSM players may also use a series of three grunts in place of a safeword during BDSM play involving gags.