A device (1) for locking a tool (2) on a machine tool comprising a first jaw (3) and a second jaw (4), which can move toward each other and away from each other, and at least one safety hook (5), which is interposed between the first jaw and the second jaw and is provided with a retention tooth (51) adapted to enter a cavity (21) provided in the shank (20) of a tool (2), characterized in that the second jaw (4) is provided with at least one seat (41) which is open toward the first jaw (3) and with a transverse bar (43) which is accommodated in said seat (41) and is rigidly associated with the second jaw (4), the safety hook (5) being supported so that it can move by the second jaw (4) and having a claw-shaped portion (50) which is directed toward the first jaw (3) and is accommodated in the space of the seat (41) that is delimited by the bar (43) and whose lower end protrudes so as to define the retention tooth (51).