We sputter-deposited Pt nanoparticles on a PdO nanoflake thin film and studied the CO sensing behavior of the Pt-decorated PdO (Pt-PdO) sensor at temperatures below 250 °C. According to X-ray photoelectron spectroscopy and temperature-programmed desorption, Pt nanoparticles can modify the electronic structure of the PdO thin film, thus improving the kinetics of PdO reduction by CO. The CO sensing response of the Pt-PdO sensor is greatly influenced by PdO reduction and reoxidation in the temperature range between 100 and 200 °C. The dramatic difference in the CO sensing behavior at 150 °C between the Pt-PdO and the bare PdO sensors is ascribed to the kinetic competition between PdO reduction and oxidation of growing Pd nanoislands on which dissociative oxygen adsorption takes place. On the other hand, the CO sensing characteristic of the Pt-PdO sensor at 100 °C and below is similar to that of the bare PdO sensor because PdO reduction is negligible in the temperature range. The Pt-PdO sensor exhibits a distinct delay in the recovery profile at 100 °C as a result of the sluggish replacement of CO adspecies by oxygen on Pt nanoparticles. At temperatures above 200 °C, the sensing behavior of the Pt-PdO sensor can be understood on the basis of the oxygen vacancy model. A highly CO-selective sensor based on the pattern recognition of the peculiar sensing response profile at 150 °C is suggested.