A power-steering device intended to be fitted on a vehicle-receiving structure, such as a cradle, the device including a rack as well as at least one actuator subassembly including at least one assistance motor, one drive gear, as well as one load-bearing structure that carries the assistance motor and is arranged to support and guide the drive gear and the rack relative to one another, the device further including one containment casing that is separate from the actuator subassembly and is fonned by connecting at least one base and one bell housing, which are attached to one another by a sealed joint such that the containment casing prevents the actuator subassembly from being exposed to water splashes, dust and salt spray from the outside environment when the power-steering device is in place on the receiving structure of the vehicle.