An outboard motor is provided with an engine holder, an engine mounted to an upper portion of the engine holder, a drive shaft housing mounted to a lower portion of the engine holder and an exhaust structure connecting an exhaust inlet opening formed to an upper surface of the engine holder to an exhaust outlet opening formed to a lower surface of the engine holder, in an installed state. The exhaust structure has an exhaust port passage extending downward from the engine and communicated with the exhaust inlet opening, an exhaust tube extending downward into the drive shaft housing and an exhaust passage formed between the exhaust inlet and outlet openings, the exhaust passage extending horizontally in the engine holder and being formed with a catalyst chamber in which a catalyst is arranged horizontally.