Abstract: An outboard motor includes a housing unit adapted to be mounted on an associated watercraft. An engine is mounted on the housing unit. The housing unit defines a water delivery passage and a water discharge passage. Both the passages communicate with each other through a lower opening. The water delivery passage is arranged to deliver cooling water to the engine. The water discharge passage is arranged to discharge the cooling water from the engine. The discharge passage communicates with a location out of the housing unit through an upper opening. A pressure relief valve unit extends through the lower and upper openings. The pressure relief valve unit allows the cooling water in the delivery passage to move to the discharge passage when a pressure of the delivery passage becomes greater than a preset pressure.