To provide a reservoir tank for a radiator of a water-cooled saddle-type vehicle which ensures an improved degree of freedom in setting the shape and mounting position of the reservoir tank. A reservoir tank for a radiator of a saddle-type vehicle includes a water reservoir section located beneath the rear part of a seat rail, for reserving the cooling water, and a water supply section extending upwardly from the water reservoir section. The reservoir tank includes a major portion of the water reservoir section located further toward an inside the vehicle than on the seat rail, and the water supply section extends upwardly from the side of the water reservoir section toward the outside of the seat rail.