An adjustment assembly of a hydraulic steering system of a vehicle, wherein the adjustment assembly is positionable between a steering column and a hydraulic steering unit of the system, wherein the adjustment assembly is suitable to adjust the mutual angular position between the rotation angle of the steering column (α) and the effective steering angle to the wheels (β), wherein said adjustment assembly comprises: - an adjustment device comprising an electric motor and an electronic command unit commanding the actuation of said electric motor; - an epicycloidal device comprising: i) a satellite-carrying member which acts as the first input being operatively connectable to the steering column; ii) at least one satellite gear supported and guided in rotation by the satellite-carrying member by means of a rotation shaft on which it is freely rotatably mounted; iii) a first solar gear which acts as an output being operatively connectable to the hydraulic steering unit, wherein said first solar gear is operatively connected to the at least one satellite; iv) a second solar gear which acts as a second input and is operatively connected to the adjustment device and to the first solar gear, through the at least one satellite, in such a way as to command the angular position and/or the rotation of the first solar gear, and therefore the output; wherein each satellite has a first toothing and a second toothing, wherein the first toothing is geared with the first solar gear, wherein the second toothing is geared with the second solar gear. - an adjustment device comprising an electric motor and an electronic command unit commanding the actuation of said electric motor; - an epicycloidal device comprising: i) a satellite-carrying member which acts as the first input being operatively connectable to the steering column; ii) at least one satellite gear supported and guided in rotation by the satellite-carrying member by means of a rotation shaft on which it is freely rotatably mounted; iii) a first solar gear which acts as an output being operatively connectable to the hydraulic steering unit, wherein said first solar gear is operatively connected to the at least one satellite; iv) a second solar gear which acts as a second input and is operatively connected to the adjustment device and to the first solar gear, through the at least one satellite, in such a way as to command the angular position and/or the rotation of the first solar gear, and therefore the output; wherein each satellite has a first toothing and a second toothing, wherein the first toothing is geared with the first solar gear, wherein the second toothing is geared with the second solar gear. i) a satellite-carrying member which acts as the first input being operatively connectable to the steering column; ii) at least one satellite gear supported and guided in rotation by the satellite-carrying member by means of a rotation shaft on which it is freely rotatably mounted; iii) a first solar gear which acts as an output being operatively connectable to the hydraulic steering unit, wherein said first solar gear is operatively connected to the at least one satellite; iv) a second solar gear which acts as a second input and is operatively connected to the adjustment device and to the first solar gear, through the at least one satellite, in such a way as to command the angular position and/or the rotation of the first solar gear, and therefore the output;