Marine heatwaves (MHWs) have devastating effects on ecosystems. Yet a global assessment of the regional impacts of MHWs on the sea-air CO2 ${\text{CO}}_{2}$ exchange is missing. Here, we analyze 30 global observation-based sea-air CO2 ${\text{CO}}_{2}$ flux data sets from 1990 to 2019. Globally, the oceanic CO2 ${\text{CO}}_{2}$ uptake is reduced by 8% (3%-19% across data sets) during MHWs. Regionally, the equatorial Pacific experiences a 31% (3%-49%) reduction in CO2 ${\text{CO}}_{2}$ release and MHWs often coincide with extreme sea-air CO2 ${\text{CO}}_{2}$ flux anomalies in this region. The oceanic CO2 ${\text{CO}}_{2}$ uptake decreases during MHWs by 29% (19%-37%) and 14% (5%-21%) in the low-to-mid latitude Northern and Southern Hemisphere, respectively. Reduced dissolved inorganic carbon in the tropics weakens outgassing, while high ocean temperatures diminish uptake in the low-to-mid latitudes. In the subpolar North Pacific and Southern Ocean, enhanced carbon uptake occurs during MHWs, but uncertainties in pCO2 ${\text{CO}}_{2}$ data sets limit a comprehensive assessment in these regions.