Abstract. Over the past decades, the role of tropical rivers in the carbon cycle paradigm has been revised due to their importance in the regional carbon budget and identified as important emitters of carbon to atmosphere. This study, conducted in the Ji-Paraná River, quantifies the CO₂ flux in the water-air interface according to the floating chamber methodology and verifies the seasonal variation and its relationship with physical, chemical and biological water parameters. The physical-chemical parameters analyzed were temperature, pH, electrical conductivity, dissolved oxygen, transparency, alkalinity, and turbidity, and the biological parameter was chlorophyll concentration. The Ji-Paraná River behaves as a source of carbon to the atmosphere, with higher emission peaks in high waters (4869.20 ± 779.95 mgC m⁻² day⁻¹) and lower values in low water (711.20 ± 131.51 mgC m⁻² day⁻¹) – the difference between the periods reached 6.8 times. Dissolved oxygen, pH, electrical conductivity and water transparency were correlated with flux and explained the gaseous exchange dynamics. CO₂ flux was strongly related to the river flow, which evidences the importance of river regime variations in gaseous emissions. Chlorophyll was strongly related to flux, therefore, the primary production is an important sink of CO₂ to the Ji-Paraná River, which indicates biological activity significantly influences the river carbon cycling.