A secondary battery system includes a secondary battery, an electric circuit, and an electronic control unit. The electronic control unit is configured to execute first calculation control, second calculation control, and estimation control when a predetermined condition is satisfied. The first calculation control is control for calculating a first impedance of the secondary battery based on a voltage value of the secondary battery at a time when a current value of the secondary battery is fluctuated on a first cycle. The second calculation control is control for calculating a second impedance of the secondary battery based on the voltage value of the secondary battery at a time when the current value of the secondary battery is fluctuated on a second cycle. The estimation control is control for estimating a reaction resistance of the secondary battery from a difference between the first impedance and the second impedance.