Abstract: The invention relates to a seal (1) for a joint comprising a cylindrical inner core layer (2) made of a water-unexpansible flexible material and having an internal opening (2a), and an outer coating layer (3) which is made of a water-expansible rubber. The outer coating layer (3) coats the inner core layer (2) and has penetrating openings (4) in the form of slits, pinholes or the like. The penetrating openings (4) extend to penetrating openings in the inner core layer (2) and communicate with the internal opening (2a) through the penetrating openings of the inner core layer (2).