In an outboard motor steering control system having an actuator steering the outboard motor, three rotation angle sensors each detecting a rotation angle of a steering wheel, three steering angle sensors each detecting a steering angle of the outboard motor, and a controller determining a drive current to be supplied to the actuator based on the detected rotation angle and steering angle and supplies the current to the actuator to control its operation, the steering angle is estimated based on the current and a detected engine speed, and the controller determines the current based on the detected rotation angle and the estimated steering angle, when all of the steering angle sensors are detected to have failed, thereby enabling steering of the outboard motor to be continued even when the steering angle sensors have all failed.