A marine vessel propulsion unit includes an engine, a drive shaft, a single propeller shaft, a single propeller, and a planetary gear mechanism. The drive shaft is arranged to extend vertically. The propeller shaft is arranged to extend in a direction intersecting the drive shaft. The rotation of the engine is transmitted to the propeller shaft via the drive shaft. The propeller is arranged to rotate together with the propeller shaft. The planetary gear mechanism is arranged at the front relative to the drive shaft and coaxial to a central rotation axis of the propeller shaft. The planetary gear mechanism is arranged to decelerate the rotation from the drive shaft and transmit the decelerated rotation toward the propeller shaft.