The electrical-accessory module (130) for engaging in an electrical box (110) includes a casing (131) that is made of insulating material and that includes: a side wall (132) that is closed at the rear by a rear wall (133) so as to define an inside space for receiving at least one electrical connection element; and an outer collar (135) that borders the side wall at the front so as to bear against the front edge (115) of the electrical box. The casing further includes a front wall (134) that closes the inside space at the front and that includes: an access window (137) for accessing each electrical connection element, the access window being closed by trim (139) that gives the electrical-accessory module its electrical function; and securing elements (138) for securing a finishing plate (150).