A servo-assisted steering device for vehicles includes a manual control member connected to a shaft driving a pressurized control fluid supplying and conveying unit, which has at least two fluid conveying lines connected to one of two chambers of one or more steering actuators. A hydraulic power unit is connected to the supplying and conveying unit and includes a reservoir for the hydraulic fluid and powered pumping means, which are driven by an electric motor and which are connected to the fluid supplying/conveying circuit to supply the pressurized fluid alternately to either one or the other of the two chambers of the actuator (or actuators) depending on the rotational direction of the control member. The electric motor is energized and/or de-energized upon operation of the control member, causing the pumping means to increase the amount of pumped fluid and reduce resistance upon steering the manual control means.