An electric power storage system for a vehicle includes an electric power storage device, a temperature sensor, and a controller. The vehicle includes an ignition switch. The electric power storage device is mounted on the vehicle. The temperature sensor is configured to detect a temperature of the electric power storage device. The controller is configured to estimate a rest temperature of the electric power storage device at a time when the ignition switch is OFF. The controller is configured to acquire the temperature of the electric power storage device at a time when the ignition switch is ON by using the temperature sensor when the ignition switch is turned ON. The controller is configured to estimate the rest temperature by using an average value of at least two of a plurality of the temperatures acquired during a predetermined period. The plurality of temperatures are the temperatures of the electric power storage device which are acquired at a time when the ignition switch is turned ON during the predetermined period.