An office chair mechanism provided with a device for adjusting the swivel force includes a fixed part secured to the chair base, movable parts secured to the seating portion and to the back rest (22) of the chair, and an elastic element which opposes the weight of the chair user, a lever having contact surfaces which interact with said fixed part by means of a fulcrum having a movable contact element interposed between the end acting on the elastic element and the other end to which the movable parts are pivoted, and an adjustment element which causes only the movable contact element to move but without modifying the position of the elastic element and of the lever.