A valve stopper for sealing a vessel opening with a valve body which may be applied to the vessel opening made from a first rigid plastic material and a valve element integrally connected to the valve body made from a second elastic plastic material is disclosed. The valve element has a sealing surface section which may be brought into contact with a support edge and a connector piece, running from the sealing surface section which may be brought into contact with a support edge and a connector piece, running from the sealing surface section to support surface of the valve body facing the vessel opening and at a distance from the interposed valve element. The sealing surface section extends over the connector piece over the whole circumference thereof, the connector piece having at least one defining recess in a given valve opening region of the sealing surface section, such that in the region of the recess the radial separation of the connector piece to the adjacent outer edge of the sealing surface section is greater than in the remaining region and the connector piece does not from a contact surface for the sealing surface section in the region of the recess and the sealing surface section has a circumferential bead on the surface thereof provided for contact with the contact edge.