A cooling system of an internal combustion engine includes an adjustment valve configured to adjust a flow rate of a cooling liquid discharged from a water jacket. A controller for the cooling system includes circuitry configured to execute flow-restriction control that controls the adjustment valve to restrict discharge of the cooling liquid from the water jacket, thereby increasing temperature of an engine body. The circuitry is configured to execute the flow-restriction control so that temperature of the cooling liquid in the water jacket at which the flow-restriction control is terminated is lower when an ambient pressure is low than when the ambient pressure is high.