A supporting structure for a cooling water reserve tank for a vehicle in which a powertrain including an engine is supported on a vehicle body by a mounting bracket and a cooling water reserve tank is attached to the mounting bracket via a reserve tank bracket. The cooling water reserve tank and cooling water stored therein is formed as a weight with the reserve tank bracket being formed as an elastic body to thereby constitute a dynamic damper, and a resonant frequency of the dynamic damper substantially coincides with a frequency of vibration occurring at a time of idling operation of the engine.