An underactuated exoskeleton for lower-limbs (1), for supporting the knee of a user, comprises a first (2) and a second coupling frame (4) and a movement means associated with the first (2) and with the second frame (4) configured to confer to the second frame (4) a roto-translation movement (RT) with respect to the first frame (2) or vice versa. The exoskeleton (1) further comprises a junction means (9) operatively interposed between the movement means and the first frame (2) and/or the second frame (4) configured to allow a relative rotation (R1) between the first (2) and the second frame (4) in a second plane (P2), orthogonal to the first plane (P1), a rotation (R) of said second frame (4) about said main axis (6) of the leg and a relative translation (T2) between the first (2) and the second frame (4) in the second plane (P2).
