A coupling system (12) between a riser (2) and an underwater supporting structure (10), includes a coupling seat (13) and a coupling head (14) which can be inserted in the seat. A pull connector (16) constrains the head (14) to the seat (13) to prevent the extraction of the head (14) but to allow rotation of the head (14). A clamping connector (18) constrains the head (14) to the seat (13) to prevent transversal translation and rotation to the head (14) and the seat (13). The pull connector (16) can be actuated alone and independently from the clamping connector (18) to make a provisional, pull-only connection between the head (14) and the coupling seat (13). The clamping connector (18) can be actuated independently from the pull connector (16) to allow postponing the complete clamping with respect to the provisional connection.