Thermal switches, which electrically turn heat flow on and off, have attracted attention as thermal management devices. Electrochemical reduction/oxidation switches the thermal conductivity (κ) of active metal oxide films. The performance of the previously proposed electrochemical thermal switches is low; the on/off κ-ratio is mostly less than 5, and the κ-switching width is less than 5 watts per meter kelvin. We used a CeO2 thin film as the active layer deposited on a solid electrolyte YSZ substrate. When the CeO2 thin film was reduced once (off-state) and then oxidized (on-state), κ was about 2.2 watts per meter kelvin in the most reduced state, and κ increased with oxidation to 12.5 watts per meter kelvin (on-state). This reduction (off-state)/oxidation (on-state) cycle was repeated 100 times, and the on/off κ-ratio was 5.8, and the κ-switching width was 10.3 watts per meter kelvin. The CeO2-based solid-state electrochemical thermal switches would be potential devices for thermal shutters and thermal displays.