A piston unit of a working cylinder has first and second coupling partners. The first coupling partner is a piston. The second coupling partner a piston rod. The piston rod is inserted axially into the piston. Each of the coupling partners has an annular groove. The groove of the piston rod is an external groove. The groove of the piston is an internal groove. The grooves of the coupling partners lie opposite one another. The unit has a coupling strand which engages in the grooves of the coupling partners and is placed into engagement by insertion into an annular groove gap defined by the grooves. The engagement of the coupling strand in the grooves produces a form-locking connection between the partners. The piston rod is oversized in relation to the piston and provides an additional frictional connection.