The invention concerns an electromechanical lock cylinder comprising a housing in which a rotatable shaft is mounted which is connected to a lock tappet to operate a locking mechanism, and at least one knob for turning said shaft. According to the invention, in a rest position, the knob is freely turnably arranged on said shaft and that electromechanically driven locking means are provided in said shaft or in said knob for, in an operating position, coupling the shaft with the knob.