A marine propulsion system that achieves both an acceleration performance and top speed closer to the performance desired by a boat driver includes an engine, propellers rotated by the driving force of the engine, a transmission mechanism arranged to convey the driving force of the engine to the propellers at least after shifting into a low speed reduction gear ratio and into a high speed reduction gear ratio, an acceleration sensor arranged to detect the acceleration of a hull propelled by the rotation of the propellers, and a control section and an ECU arranged to carry out the control for changing the reduction gear ratio of the transmission mechanism. The control section and the ECU are configured to control the transmission mechanism to shift from the low speed reduction gear ratio into the high speed reduction gear ratio based on the acceleration of the hull.