The device includes a tubular fitting (6) that is part of a bicycle frame and able to accommodate a saddle post (7), elements (19,20) for locking the saddle post (7) in rotation, and members (8) for holding the saddle post (7) in a longitudinal position that is determined relative to the tubular fitting (6). The holding members comprise a stop surface (7a) that is provided on the saddle post (7) and able to work with a stop (8) that is provided on the fitting (6) so as to delimit the insertion length of the saddle post (7) in the latter. The holding members also include at least one annular crosspiece (9, 10) that can be arranged between the stop surface and the stop so as to allow the adjustment of the insertion length.