A large area including the central-northern part of Argentina, southern Bolivia, central Chile, and most of Paraguay and Uruguay, experienced record-breaking temperatures during two consecutive heatwaves in late November and early December 2022. During the second heatwave, nine locations in northern Argentina registered their highest maximum temperature of December since at least 1961. Our analysis based on observational and reanalysis datasets indicate that South America, like the rest of the world, has experienced heatwaves increasingly frequently in recent years. The December 2022 heatwave has an estimated return time of 1 in 20 years in the current climate, meaning it has about a 5% chance of happening each year. To estimate how human-caused climate change has influenced the likelihood and intensity of the observed heatwave, we combined climate models with the observation-based data. We found that human-caused climate change made the event about 60 times more likely. A heatwave with a return period of 20 years would be about 1.4 °C less hot in a world without anthropogenic global warming. Heatwaves this early in the summer season pose a substantial risk to human health and are potentially lethal. This risk is aggravated by climate change, but also by other factors such as an aging population, urbanisation and the built environment, and individual behavior and susceptibility to the heat. This highlights the importance of attribution studies in a region already threatened and vulnerable to climate change.