A gag is usually an item or device designed to prevent speech, often as a restraint device to stop the subject from calling for help and keep its wearer silent. This is usually done by blocking the mouth, partially or completely, or attempting to prevent the tongue, lips, or jaw from moving in the normal patterns of speech. The more "effective" a gag appears to be, the more hazardous it is. For example duct tape is a fairly effective method of keeping a person's mouth shut but can be hazardous if the subject cannot breathe freely through the nose (for example if they have the common cold). For this reason, a gagged person should never be left alone.

The use of gags is commonly depicted in soap operas and crime fiction, particularly in comics, novels and films.

Courts have been known to gag certain people, such as the civil rights activist Bobby Seale. This practice has been criticized as inhumane.