<p>CO<sub>2</sub> efflux at the water–air interface is an essential component of the riverine carbon cycle. However, the lack of spatially resolved CO<sub>2</sub> emission measurement still hinges the accuracy of estimates on global riverine CO<sub>2</sub> emissions. By deploying floating chambers, seasonal changes in river water CO<sub>2</sub> partial pressure (<i>p</i>CO<sub>2</sub>) and CO<sub>2</sub> evasion from the Dongjiang River in South China were investigated. Lateral soil CO<sub>2</sub> input and dilution effect caused by precipitation played critical roles in controlling riverine <i>p</i>CO<sub>2</sub> in small rivers, while the decomposition of allochthonous organic carbon is responsible for <i>p</i>CO<sub>2</sub> variability in large rivers. Temperature-normalized gas transfer velocity (<i>k</i><sub>600</sub>) in small rivers were 8.29 ± 11.29 m d<sup>−1</sup> and 4.90 ± 3.82 m d<sup>−1</sup> for the wet season and dry season, respectively, which were nearly 70 % higher than that of large rivers (3.90 ± 5.55 m d<sup>−1</sup> during the wet season and 2.25 ± 1.61 m d<sup>−1</sup> during the dry season). A significant correlation was observed between <i>k</i><sub>600</sub> and flow velocity but not wind speed regardless of river size. Majority of the surveyed rivers were net CO<sub>2</sub> source, exhibiting substantial seasonal variations. The mean CO<sub>2</sub> flux was 300.1 and 264.2 mmol m<sup>−2</sup> d<sup>−1</sup> during wet season for large and small rivers, respectively, 2-fold larger than that during dry season. The absence of commonly observed higher CO<sub>2</sub> fluxes in small rivers could be associated with the depletion effect caused by abundant and consistent precipitation in this subtropical monsoon catchment.</p>