Tool for injection-molding a hearing protection cup which comprises a curved shell (2) with an opening for receiving an ear, and a washer (7) which extends around the opening and is connected to the edge thereof. The washer projects inward across the opening along at least two opposite sides thereof and is intended to support a sealing and comfort ring which is intended to rest against the head. The tool comprises a comon mold cavity intended to produce the shell (2) and the washer (7) in one piece, and an insert which can be introduced into this mold cavity for forming the inner surfaces of the shell and of the washer. The insert is divided into at least three portions (13, 14) by means of dividing planes which extend in the same principal direction as the opposite sides of the opening. The invention also relates to a method for producing a hearing protection cup, and to a hearing protection cup produced using said method.