A power source system for a vehicle includes a first power storage device, a second power storage device, a voltage converter, and a controller. The controller is configured to (a) execute a charging control in which the second power storage device is charged by the voltage converter when a parking time of the vehicle exceeds a predetermined period of time, (b) estimate a decrease amount in a state of charge of the first power storage device in a time period from after elapse of the predetermined period of time until a start of the vehicle, (c) set a lower limit of the state of charge based on the decrease amount, and (d) not execute the charging control when the state of charge is lower than the lower limit.