Catalytic oxidation of formaldehyde (HCHO) to CO2 and H2O under ambient condition is highly desired for purifying indoor air quality. In this work, it was found for the first time that TiO2 supported subnanometer Rh catalyst exhibited a remarkable activity with complete removal of HCHO at room temperature. The humidity under ambient condition can significantly improve the activityi stability and specific rates but result in lower activation energy for the oxidation of HCHO on this catalyst. A combination of characterizations, such as H-2 temperature programmed reduction, adsorption microcalorimetry and in situ diffuse reflectance infrared Fourier transform spectroscopy, demonstrated the importance of Rh species for the dissociative adsorption of O-2 and the key role of water on the oxidation of HCHO. The O atoms facilitated the transformation of HCHO adsorbed on TiO2 into intermediate of formates, while the presence of water led to the production of hydroxyl probably from the reaction with the adsorbed O species, which promoted the decomposition of formates to CO2. This study can provide an important implication into the design and fabrication of more economic Rh-based catalysts for ambient HCHO oxidation.