The seal for aircraft boarding bridge, comprises a bellows (5) that spans a gangway bottom plate (10), which comprises a flexible frame (3) at its one open end and having at its other end a gantry frame for fastening to the aircraft to the boarding bridge, the flexible frame (3) resting against the outer skin of the aircraft when the bellows is deployed and an actuation device for actuating the bellows being provided for extending and retracting the bellows. The actuation device comprises a rotatable shaft equipped with a drive. The rotatable shaft (40) permits winding and unwinding of a traction mechanism connected to the flexible frame, the actuation device (20) comprising two articulated arms (30) arranged in a spaced-apart relationship, each articulated arm (30) comprising an upper arm member (31) and a lower arm member (35) coupled to said upper arm member by a hinge, said upper arm member (31) of the articulated arm (30) being hinged to the gantry frame (2) in a roof region of the bellows (5) and the lower arm member (35) being hinged to the flexible frame (3) and an extension mechanism (51, 55) being provided for extending the bellows (5).