<p>This study reports the surface water partial pressure of CO₂ (<i>p</i>CO₂) and air-sea CO₂ fluxes on the East China Sea shelf off the Changjiang estuary in August 2023. Surface water <i>p</i>CO₂ ranged from 110 μatm to 910 μatm with an average value of 427±154 μatm. Air-sea CO₂ fluxes in the surveyed area ranged from −20.6 mmol m⁻² d⁻¹ to 35.9 mmol m⁻² d⁻¹ and averaged 3.0±8.9 mmol m⁻² d⁻¹ (a moderate source), which was contrary to this region generally being a CO₂ sink during summer. Changjiang discharge played a key role in regulating surface water <i>p</i>CO₂; the decreased Changjiang discharge in August 2023 increased surface water <i>p</i>CO₂ on the adjacent inner shelf substantially, and the high sea surface temperature further elevated the surface water <i>p</i>CO₂. The combined effect of drought and high temperatures in August 2023 turned the study area from a CO₂ sink to a CO₂ source. Under the context of global change, climate events such as floods, droughts and heatwaves occur more frequently, which will continue to add more complexity to CO₂ sink/source evaluations in large river-dominated marginal seas and suggest further research is needed.</p>