There is provided a radiator reservoir tank including: a tank body that is connected to a radiator and that includes an inlet section and an accumulation section for cooling water; a first rib that is disposed inside the tank body, and that extends out from a side wall at one side of facing side walls of the tank body toward another side, and that partitions between the inlet section and the accumulation section, and that is provided with a gap to the side wall at the other side; and a second rib that is provided further toward the inlet section side or the accumulation section side than the first rib, that extends out from the side wall at the other side toward the one side, and that is provided with a gap to the side wall at the one side.