The electromechanical cylinder is of the type which is formed by means of a knob (4) fixed to the cylinder body (5) mounted in the respective lock, connected to the outer face of the latter, namely it includes a single knob, with the particular feature that the rotor (6), which together with the knob (4) forms an electronic module, is situated inside a housing (15) of the cylinder body (5) and is retained inside the latter axially, but with the possibility of rotating freely so that, when the rotor (6) is mounted on the knob (4), it remains on the inside and inaccessible so as to prevent any tampering therewith, which is due to the fact that a base (7), which forms the element for fixing together the knob (4) and the cylinder body (5), is fixed by means of screws (38) which, when the assembly is mounted, are positioned facing each other in the vicinity of the associated door on which the lock is located, preventing said possible tampering or forcing.