The adjustment device according to the invention for an adjustable element, in particular the backrest of a chair, has a guide rail and a sliding element which is movable in the guide rail along an adjustment axis. Furthermore, a blocking element is provided, which blocking element can be secured on the guide rail or on the sliding element in a latching position in which the sliding element can be moved along the adjustment axis at least in one direction between two end positions, wherein the blocking element comes into operative contact with latches spaced apart along the adjustment axis. During the installation of the adjustment device, the blocking element is secured on the guide rail or the sliding element in an installation position in which the sliding element can be introduced into the guide rail in the direction of the adjustment axis.