An underactuated exoskeleton for lower-limbs, for supporting a knee of a user, comprising: - a first coupling frame provided with upper attaching means configured to be joined to a thigh of a lower limb of a user; - a second coupling frame provided with lower attaching means configured to be joined to a corresponding leg of said lower limb of said user, said second coupling frame having a main axis that, in use, corresponds to a main axis of said leg; - movement means associated with said first and said second frame and configured to give said second frame a roto-translation movement with respect to the first frame or vice versa; wherein said movement means comprise: a rotary actuation unit, a motion transmission unit operatively interposed between said rotary actuation unit and said first or second frame configured to transform a twisting moment imparted by said rotary actuation unit into a first roto-translation of the first or second frame in a sagittal plane of said knee; characterised in that it comprises joint means operatively interposed between said movement means and said first frame and/or said second frame and provided with: a first rotational joint, configured to allow a relative rotation between said first and said second frame about an axis lying in a frontal plane, orthogonal to said sagittal plane; a second rotational joint, configured to allow a rotation of said second frame about its main axis; a third joint, rotatably or linearly yieldable in said frontal plane, to allow, in combination with said first joint and with said transmission unit, a relative translation between said first and second frame along an axis lying in said frontal plane. - a first coupling frame provided with upper attaching means configured to be joined to a thigh of a lower limb of a user; - a second coupling frame provided with lower attaching means configured to be joined to a corresponding leg of said lower limb of said user, said second coupling frame having a main axis that, in use, corresponds to a main axis of said leg; - movement means associated with said first and said second frame and configured to give said second frame a roto-translation movement with respect to the first frame or vice versa; wherein said movement means comprise: a rotary actuation unit, a motion transmission unit operatively interposed between said rotary actuation unit and said first or second frame configured to transform a twisting moment imparted by said rotary actuation unit into a first roto-translation of the first or second frame in a sagittal plane of said knee; a rotary actuation unit, a motion transmission unit operatively interposed between said rotary actuation unit and said first or second frame configured to transform a twisting moment imparted by said rotary actuation unit into a first roto-translation of the first or second frame in a sagittal plane of said knee; a first rotational joint, configured to allow a relative rotation between said first and said second frame about an axis lying in a frontal plane, orthogonal to said sagittal plane; a second rotational joint, configured to allow a rotation of said second frame about its main axis; a third joint, rotatably or linearly yieldable in said frontal plane, to allow, in combination with said first joint and with said transmission unit, a relative translation between said first and second frame along an axis lying in said frontal plane.