A ship propulsion unit (10, 10A) attachable to an outside of a ship on a rear end side of a ship body, the ship propulsion unit (10, 10A) comprising: an engine including a crankshaft; a propeller shaft disposed below the engine with respect to an up-down direction of the ship and having one end provided with a propeller; a power transmission mechanism (60, 60A) configured to couple an output portion of the crankshaft and a coupling portion at an other end of the propeller shaft, and transmit driving force of the engine to the propeller shaft; an upper case (30, 30A) accommodating the output portion of the crankshaft and an upper portion of the power transmission mechanism (60, 60A); and a lower case connected to a lower portion of the upper case (30, 30A) with respect to the up-down direction of the ship, accommodating the propeller shaft such that the propeller is disposed outside, and accommodating a lower portion of the power transmission mechanism (60, 60A), the power transmission mechanism (60, 60A) including a drive shaft (65, 65A) that extends from the upper case (30, 30A) toward the lower case, the upper case (30, 30A) and the engine being designed to be fixed to the ship body, a connection portion between the upper case (30, 30A) and the lower case being provided with a rotation mechanism configured to rotate the lower case relative to the upper case (30, 30A) around an axis extending in an extending direction of the drive shaft (65, 65A), characterized in that, when mounted on a ship, the crankshaft extends in a front-rear direction of the ship and the output portion of the crankshaft extends from the engine toward the rear end side of the ship body. an engine including a crankshaft; a propeller shaft disposed below the engine with respect to an up-down direction of the ship and having one end provided with a propeller; a power transmission mechanism (60, 60A) configured to couple an output portion of the crankshaft and a coupling portion at an other end of the propeller shaft, and transmit driving force of the engine to the propeller shaft; an upper case (30, 30A) accommodating the output portion of the crankshaft and an upper portion of the power transmission mechanism (60, 60A); and a lower case connected to a lower portion of the upper case (30, 30A) with respect to the up-down direction of the ship, accommodating the propeller shaft such that the propeller is disposed outside, and accommodating a lower portion of the power transmission mechanism (60, 60A), the power transmission mechanism (60, 60A) including a drive shaft (65, 65A) that extends from the upper case (30, 30A) toward the lower case, the upper case (30, 30A) and the engine being designed to be fixed to the ship body, a connection portion between the upper case (30, 30A) and the lower case being provided with a rotation mechanism configured to rotate the lower case relative to the upper case (30, 30A) around an axis extending in an extending direction of the drive shaft (65, 65A), characterized in that, when mounted on a ship, the crankshaft extends in a front-rear direction of the ship and the output portion of the crankshaft extends from the engine toward the rear end side of the ship body.