An embodiment of a Geiger-mode avalanche photodiode includes a body of semiconductor material having a first conductivity type, a first surface and a second surface; a trench extending through the body from the first surface and surrounding an active region; a lateral-isolation region within the trench, formed by a conductive region and an insulating region of dielectric material, the insulating region surrounding the conductive region; an anode region having a second conductivity type, extending within the active region and facing the first surface. The active region forms a cathode region extending between the anode region and the second surface, and defines a quenching resistor. The photodiode has a contact region of conductive material, overlying the first surface and in contact with the conductive region for connection thereof to a circuit biasing the conductive region, thereby a depletion region is formed in the active region around the insulating region.