A rubber dam for isolating one or more teeth in a patient's mouth includes an annular perioral frame (2) to be arranged extraorally, an elastic and film-like cover (1) connected to the frame (2) and which in the untensioned state has a bag-shaped section (3) for insertion into the oral cavity, and a fastener (6) for the intraoral fastening of the cover (1). The fastener (6) is formed by an elastic annular element (6) insertable into the vestibule (8) in an elastically deformed state and then exerts a force on the cover (1) pushing and firmly retaining it in the vestibule (8). When the rubber dam is inserted, the cover (1) extends in a slightly tensioned state between the frame (2) and the fastener (6) located in the vestibule (8) around the corners of the mouth and the lips (7) and intraorally in the upper and lower jaw in a slightly tensioned state out of the vestibule (8), over the front sides of the teeth and in the palate and tongue area in the plane of the chewing surfaces, and pushes the lips, the corners of the mouth and the cheeks away from the teeth as well as the jaws apart, assisting the opening of the mouth.