An outboard motor steering system for an outboard motor mounted on a stern of a boat and having an internal combustion engine at its upper portion and a propeller with a rudder at its lower portion powered by the engine to propel and steer the boat. The system includes a swivel shaft connected to the propeller to turn it relative to the boat, a swivel case fixed to the outboard motor and rotatably accommodating the swivel shaft, and a hydraulic actuator connected to the swivel shaft to rotate it. The actuator has a shape whose height is larger than its width and is installed in such a manner that a direction of the height is in parallel with a vertical direction, so as not to project outside a profile of the outboard motor, obtained by looking down the outboard motor from downward, regardless of a steered angle of the outboard motor. Specifically, the actuator is two hydraulic cylinders installed in an over-under manner or an elliptic hydraulic cylinder having a plurality of piston rods installed in the same manner. With this, it can enhance the output of the actuator to increase the steering force (torque), can prevent the actuator from projecting outside the profile, and can avoid constriction of the space around the outboard motor.