A vehicle power source system includes a plurality of module groups each including a plurality of battery modules and disposed separately from each other, and a cooling circuit including a plurality of module group cooling units which are configured to respectively cool the plurality of module groups. The plurality of module group cooling units are connected in series. Each of the plurality of module group cooling units includes a plurality of module cooling units, each of which is configured to cool one or more of the battery modules. In each of the plurality of module group cooling units, the plurality of module cooling units are connected in parallel.