A plastic outlet valve for containers for liquids has a valve housing having an inlet socket and a union nut for securing the inlet socket on an outlet socket of a container. An anti-twist device in the form of an outer annular cam arrangement with locking cams is arranged on the inlet socket of the valve housing. The locking cams interlock with locking cams of an inner annular cam arrangement of the outlet socket of the container. As an alternative, an adapter ring with inner and outer locking cams is provided. The adapter ring is slipped onto the insertion end of the inlet socket of the valve housing. The inner locking cams engage matching external longitudinal grooves of the inlet socket of the valve housing, and the outer locking cams engage matching inner longitudinal grooves provided on the outlet socket of the container.