A connecting device for a medical infusion system includes a connection piece which has a connecting profile for the connection of a functional part of the infusion system. The connecting device also has a resiliently flexible, cup-like valve body which is arranged in the connection piece and has a valve casing and cap-shaped top region which is provided with a slit arrangement. Further, the connecting device has a dimensionally stable base portion on which a base ring of the valve casing is supported and which is firmly connected to the connection piece. In the unloaded initial state, the valve casing has a convex internal contour which, starting from the cap-shaped top region, first expands in the direction of the base ring and subsequently narrows again, forming an O-like internal longitudinal section. The connecting device can be in the form of a three-way stopcock for an infusion system.
