A fuel tank for a personal watercraft of the type in which a main body portion of the fuel tank is disposed in a watercraft body. An oil supply port member of the fuel tank is provided at a deck constituting an upper portion of the watercraft body. The main body portion and the oil supply port member are connected to each other by a filler hose. The oil supply port member is closed with an oil supply cap. The oil supply port member includes a cylindrical passage provided with an oil supply port at one end thereof and with a hose connection portion at the other end thereof. A stopper for receiving the tip end of an oil supply nozzle is provided at an intermediate portion of the passage. The supply nozzle for re-supplying the fuel can be prevented from penetrating into the main body portion to a needlessly large extent. As a result, the main body portion of the fuel tank can be prevented from suffering scratches or the like.