Ultrafast detection of dynamic variations in the carbon dioxide (CO2) gas concentration with good sensitivity is crucial in many applications that range from respiration monitoring to fire detection. However, the chemical inertness of the CO2 sensor makes its detection with high sensitivity quite difficult, and only a few materials were reported that can sense CO2 effectively. Nevertheless, the sensors based on these materials exhibited very low sensitivity with large response times, and thus they are not suitable for many practical applications. Here, we report a highly sensitive, reversible, and ultrafast detection of CO2 gas in air using a resistive gas sensor based on ZnO nanoflakes. Excellent sensitivity (0.1125 ppm-1 for 600 ppm) with ultrafast response (<20 s) is observed upon exposure of the sensor to 200-1025 ppm of CO2 at 250 °C. The sensing mechanism of the device is explained by the oxygen vacancy model. Further, the effect of the temperature and cross-sensitivity of the sensor to other gases were experimentally investigated. Being highly sensitive and faster, this CO2 sensor can be utilized in numerous applications where high response and recovery times along with good sensitivity are extremely important. © 2019 American Chemical Society.