Compared to conventional hybrid electric vehicles, plug-in hybrid vehicles have a larger-capacity battery and an onboard charger. These devices are mounted in functionally optimal locations, so it is a challenge to provide a thermal management system that achieves a good balance between high cooling performance and low cost. The battery should be operated at required temperature to secure safety and durability at high temperatures, and to suppress drops in output and capacity. However, setting separate cooling systems suited for each device leads to both an increased cost and weight. Therefore, an integrated water cooling system was devised for the battery, charger, and DC-DC converter, and the cooling performance was verified through simulations and tests. A valve installed before the battery in the cooling circuit allows it to be bypassed when coolant temperature rises due the charger or low–speed engine operation, helping to preserve battery life. Conversely, this waste heat can be used to heat up the battery when it is cold. In addition, the target thermal resistances of all cooling components were appropriately determined so that the battery temperature does not exceed the upper limit even in high ambient temperature conditions. As a result, tests showed that high power continuous driving could be performed, using only radiator cooling, and without a drop in battery output, even in a hot environment. The above results demonstrated that the developed integrated water cooling system satisfies the thermal management performance in all PHEV operation modes. The developed cooling system was used in the CLARITY PLUG-IN HYBRID and CLARITY ELECTRIC, which is the first adoption of a battery water cooling system in a Honda vehicle.