Apparatus for unwinding flexible cables on seabeds, which comprises a support structure (2) for a reel (5) on which a flexible cable (6) is wound, provided with two shoulders (4) each advantageously formed by two counter-facing towers (19), between which a seat (20) is defined susceptible for slidably receiving a hub-carrying group (7). Each shoulder (4) is associated with a plurality of aligned anchoring elements (12), a slidably movable reaction bar (13) guided in the aforesaid seat (20), and first and second locking means (14, 15), each mounted on the hub-carrying group (7) and on the reaction bar (13), able to be actuated to move between an interference position (A, A′), in which they intercept at least one anchoring element (12) constraining the hub-carrying group (4) or the reaction bar (13) to the relative shoulder (4), and a first non-interference position (B, B′), in which they are released from the anchoring elements (12), freeing the hub-carrying group (7) or the reaction bar (13) from the relative shoulder (4). Two first movement actuators (16), each of which mounted between a hub-carrying group (7) and the corresponding reaction bar (13), allow modifying the distance between them with the first or the second locking means (14, 15) in non-interference position (B, B′).