Climate change is leading to increased frequency and magnitude of heatwaves across global ecosystems. The extent to which heatwave events affect ecosystem carbon balances is uncertain. Here, we used extensive data from a global network of eddy covariance (EC) flux tower sites to identify heatwave events within the temporal range of the available data. We then examined the effects of heatwaves on plant carbon uptake and ecosystem respiration using data derived from the EC measurements and at selected sites using a well-tested processed based model (ecosys). Our results show contrasting ecosystem carbon cycle responses during heatwaves in various biomes of the global ecosystem. We show that heatwaves affect gross primary production (GPP) and ecosystem respiration (RE), depending on ambient climate and site properties. On average, compared to the pre-heatwave period, ecosystems experienced a reduction of 82% and 65% in observed and modeled net ecosystem productivity (NEP), respectively. These NEP reductions continued even after the heatwave events, with a decrease of ~34% in both EC and modeled NEP in the subsequent months. The NEP declines were primarily attributed to GPP decreases and slight RE increases in many of the EC sites around the world. Forested sites, on average, had larger NEP reductions during heatwaves compared to non-forested sites. However, forested sites remained carbon sinks during the heatwave and post-heatwave periods, indicating their resilience to these short-term events. In contrast, our modeling results suggest that non-forested ecosystems experienced increased carbon losses due to rapid leaf-off, caused by reduced carbon assimilation and elevated maintenance respiration, leading to senescence. We conclude that expected increases in extreme heatwave events affect a complex set of ecosystem processes that reduce net ecosystem carbon uptake, resulting in a positive feedback to climate change.