A device (10) for locking unicortical pins, comprising: a pin-locking arm (101) provided with two seats (101a, 101b) suitable for locking a corresponding number of unicortical pins (100); and a connection base (102), intended to couple with a connecting body (11) of an anchoring group (20) for an external fixator (1), locking means (103) configured to act simultaneously on the seats (101a, 101b), locking in position two unicortical pins (100) inserted therein, wherein said locking means (103) comprise deformable spheres (103a, 103b) which are passed through by an insertion channel (1031) for the unicortical pins (100), and a pressure plate (103c) acting on the seats (101a, 101b) for compress both said deformable spheres (103a, 103b) locking in position the unicortical pins (100) inserted in the respective insertion channels (1031).