Owing to the urgency of the climate change crisis and the emerging greenwashing issue that companies face, greenwashing in companies' carbon reduction is exceptionally important in the progress of global carbon reduction and the achievement of the Paris Agreement goals. Therefore, this study focused on multinational corporations (MNCs) listed on the 2021 Fortune Global 500 list (2001–2020) and used institutional theory to theoretically discuss and empirically test the relationship between MNCs' environmental scores and carbon emissions/intensity, as well as the moderating effect of national culture. Our results demonstrate that the environmental scores of MNCs positively affect their carbon emissions and intensity, meaning that managers tend to use high scores to hide their poor carbon reduction, showing a typical “talk” more and “act” less (i.e., greenwashing) behavior. Meanwhile, the moderating effects of individualism, masculinity, and uncertainty avoidance were negative. These revealed that the balance of MNCs' “talk” and “act” can be influenced by the domestic national cultural characteristics. Further analysis showed that the Paris Agreement did not prevent MNCs' greenwashing in carbon reduction. Such greenwashing behavior has become even worse in societies with power distance characteristics after the Paris Agreement. To the best of our knowledge, this is the first study to investigate companies' greenwashing in terms of carbon reduction. The goal is to enhance our understanding of why and how greenwashing occurs based on the institutional theory perspective, extend the application of institutional theory, enrich the literature on companies' greenwashing and national culture, and offer insight into interdisciplinary research between national culture and green transition management. Finally, suggestions for policymakers on how to prevent MNCs' greenwashing in carbon reduction, considering national cultural influence, were provided.