The invention relates to a cable-tensioning device associated with a rail (3) having at its ends (4, 5) two points for returning at least one cable (8), characterized in that one (4) of the said ends of the rail includes: a rotary cam (12); a cam-support element (7); a spring (13) coaxial with the cam (12) and having two angularly offset radial tabs (32, 33), a first tab (33) bearing on a stop (19) formed on the support element (7), and the second tab (32) extending inside the cam (12) and being designed to slide over a non-return means (20) arranged on the support element (7) after the cam has rotated through a first travel, this non-return means being shaped to prevent any return of the second tab (32) after the tabs of the spring (13) have become angularly separated by at least the length of the arc between the non-return means (20) and the stop (19), the cable (8) being engaged in grooves (30, 31) formed at the periphery of the cam. This invention has application to motor vehicle door window lifters.