The Red River (Vietnam) is representative of a south-east Asian river system, strongly affected by climate and human activities. This study aims to quantify the spatial and seasonal variability of CO2 partial pressure and CO2 emissions of the lower Red River system. Water quality monitoring and riverine pCO2 measurements were carried out for 24 h at five stations distributed along the lower Red River system during the dry and the wet seasons. The riverine pCO2 was supersaturated relative to the atmospheric equilibrium (400 ppm), averaging about 1589 ± 43 ppm and resulting in a water–air CO2 flux of 530.3 ± 16.9 mmol m−2 d −1 for the lower Red River. pCO2 and CO2 outgassing rates were characterized by significant spatial variation along this system, with the highest values measured at Hoa Binh station, located downstream of the Hoa Binh Dam, on the Da River. Seasonal pCO2 and CO2 outgassing rate variations were also observed, with higher values measured during the wet season at almost all sites. The higher river discharges, enhanced external inputs of organic matter from watersheds and direct inputs of CO2 from soils or wetland were responsible for higher pCO2 and CO2 outgassing rates. The difference in pCO2 between the daytime and the night-time was not significant, suggesting weak photosynthesis processes in the water column of the Red River due to its high sediment load.