An outboard motor is equipped with an engine, an engine holder, an oil pan disposed below the engine in a state of the outboard motor being mounted to a hull, a water pump disposed below the oil pan, and a cooling water passage structure. The cooling water passage structure includes a vertical cooling water passage vertically passing through inside the oil pan and communicated with the side of the engine, a lateral cooling water passage extending in a lateral direction along a bottom surface of the oil pan, a cooling water supply pipe extending upward from the water pump and connected to a side of the engine, and a water pressure relief valve provided for the lateral cooling water passage for controlling a pressure increasing of the cooling water. The lateral cooling water passage has one end communicated with a lower end of the vertical cooling water passage and has another one end to which an upper end of the cooling water supply pipe is connected.