An adjustment device for adjustment the inclination of the backrest of a seat of a vehicle, comprising: - a first and a second adjustment element (24, 26), rotatable relative to one another about a longitudinal axis and constrained at a relative fixed distance in the direction of said longitudinal axis, the first and the second adjustment element (24, 26) having respective cylindrical walls (28, 30) overlapped to each other and respectively provided with a first array of slots and a second array of slots inclined to each other, - a control pin rotatable about said longitudinal axis, comprising a threaded adjustment portion and a fastening portion, wherein the fastening portion is fixed to a ring nut to constrain the control pin in fixed axial position with respect to said first and second adjustment element (24, 26), - a central body housed between said first and second adjustment element (24, 26) and having a threaded central hole engaged with said threaded adjustment portion of said control pin, and wherein the central body is movable axially along said longitudinal axis following a rotation of said control pin about the longitudinal axis, - a plurality of radial pins carried by the central body and movable radially with respect to the central body between an extracted position and a retracted position and cooperating with elastic means tending to push said radial pins towards said extracted position, wherein in the extracted position each of said radial pins engages a first slot and a second slot of said adjustment elements (24, 26) and in the retracted position said pins are disengaged from the first array of slots, - at least one disengagement element rotatable about said longitudinal axis independently of said first and second adjustment element (24, 26) and from said central body and having a plurality of cams adapted to control the movement of said radial pins from the extracted position to the retracted position as a result of an angular movement between the disengagement element and the central body, and - a release element (22, 84) for imparting an angular movement about said longitudinal axis to said disengagement element. - a first and a second adjustment element (24, 26), rotatable relative to one another about a longitudinal axis and constrained at a relative fixed distance in the direction of said longitudinal axis, the first and the second adjustment element (24, 26) having respective cylindrical walls (28, 30) overlapped to each other and respectively provided with a first array of slots and a second array of slots inclined to each other, - a control pin rotatable about said longitudinal axis, comprising a threaded adjustment portion and a fastening portion, wherein the fastening portion is fixed to a ring nut to constrain the control pin in fixed axial position with respect to said first and second adjustment element (24, 26), - a central body housed between said first and second adjustment element (24, 26) and having a threaded central hole engaged with said threaded adjustment portion of said control pin, and wherein the central body is movable axially along said longitudinal axis following a rotation of said control pin about the longitudinal axis, - a plurality of radial pins carried by the central body and movable radially with respect to the central body between an extracted position and a retracted position and cooperating with elastic means tending to push said radial pins towards said extracted position, wherein in the extracted position each of said radial pins engages a first slot and a second slot of said adjustment elements (24, 26) and in the retracted position said pins are disengaged from the first array of slots, - at least one disengagement element rotatable about said longitudinal axis independently of said first and second adjustment element (24, 26) and from said central body and having a plurality of cams adapted to control the movement of said radial pins from the extracted position to the retracted position as a result of an angular movement between the disengagement element and the central body, and - a release element (22, 84) for imparting an angular movement about said longitudinal axis to said disengagement element.