A fuel tank assembly includes a fuel tank disposed substantially along a seat rail in a space surrounded by a lower portion of a seat and the seat rail of a motorcycle, and a fuel pump unit disposed in a lower portion of the fuel tank. A mounting flange for mounting the fuel pump unit to the fuel tank is arranged below the seat rail and forward of a front end of a rear wheel of the motorcycle as viewed in a side view of the motorcycle. The mounting flange of the fuel pump unit is disposed above a side frame of a body frame of the motorcycle. Such fuel tank assembly provides the degree of freedom of seat layout, and does not interfere with a rear wheel of motorcycle, or gravel or the like thrown up by the rear wheel, during operation the motorcycle.