A vessel propulsion system includes an engine that provides a propulsive force to a vessel, a throttle valve to adjust an air amount supplied to the engine, an accelerator operation unit operated by an operator to adjust a throttle opening degree, the throttle opening degree corresponding to an operation amount of the accelerator operation unit, and a control unit programmed to vary an engine output even when the throttle opening degree is maintained at a fixed value other than an idling opening degree, the engine output corresponding to the operation amount of the accelerator operation unit as a reference output.