A marine propulsion device includes a first gear, a second gear, a third gear, a circulator, a case, and an attachment body. The second gear is meshed with the first gear. The third gear is meshed with the first gear, and the third gear is coaxial with the second gear and opposed to the second gear. The circulator is between the second gear and the third gear. The case includes an internal space in which the first gear, the second gear, the third gear, and the circulator are located. The case includes an attachment hole to which the circulator is attached. The attachment body is inserted into the attachment hole so as to attach the circulator to the case.