The electromechanical cylinder is of the kind consisting of a knob (4) fixed to the cylinder body (5) fitted in the respective lock, in correspondence with the outer face of the latter, in other words it includes a single knob, with the particular feature that the rotor (6) which, together with the knob (4), form an electronic module, is located in a housing (15) of the cylinder body (5) and is retained therein axially but with the possibility of rotating freely, in such a way that in fitting the rotor (6) on the knob (4) the former remains in the inner and inaccessible part in order to prevent any manipulation thereof, this being a consequence of the fact that a base (7) forming the fastening element between the knob (4) and the cylinder body (5) is fixed by means of screws (38) which, in the assembly of the unit, remain facing in proximity to the actual door in which the lock is located, preventing such possible manipulations or violent acts.