A plunger (1) for a medical delivery device comprises: a rod element (2) having a distal end (22) and a proximal end, and a stopper (3) mounted to the rod element (2) at its distal end (22). The rod element (2) has a first form fit structure (24) and the stopper (3) has a second form fit structure (31). The stopper (3) is locked to the rod element (2) by the first form fit structure (24) of the rod element (2) and the second form fit structure (31) of the stopper (3) engaging each other.