A marine vessel propulsion unit includes an engine, a drive shaft, a propeller shaft, a propeller, an intermediate shaft, and a first reduction gear mechanism. The drive shaft is arranged to be rotated by the engine. The rotation of the drive shaft is transmitted to the propeller shaft. The propeller is arranged to be rotated together with the propeller shaft. The intermediate shaft is arranged on a central rotation axis of the propeller shaft or an extension of the central rotation axis. The intermediate shaft is arranged to transmit rotation between the drive shaft and the propeller shaft. The first reduction gear mechanism is arranged on a central rotation axis of the propeller shaft or the extension of the central rotation axis. The first reduction gear mechanism is arranged to decelerate the rotation of the intermediate shaft so as to transmit the decelerated rotation to the propeller shaft during both forward propulsion and backward propulsion.