Adaptation is key to minimizing heatwaves' societal burden; however, our understanding of adaptation capacity across the socioeconomic spectrum is incomplete. We demonstrate that observed heatwave trends in the past four decades were most pronounced in the lowest-quartile income region of the world resulting in >40% higher exposure from 2010 to 2019 compared to the highest-quartile income region. Lower-income regions have reduced adaptative capacity to warming, which compounds the impacts of higher heatwave exposure. We also show that individual contiguous heatwaves engulfed up to 2.5-fold larger areas in the recent decade (2010-2019) as compared to the 1980s. Widespread heatwaves can overwhelm the power grid and nullify the electricity dependent adaptation efforts, with significant implications even in regions with higher adaption capacity. Furthermore, we compare projected global heatwave exposure using per-capita gross domestic product as an indicator of adaptation capacity. Hypothesized rapid adaptation in high-income regions yields limited changes in heatwave exposure through the 21st century. By contrast, lagged adaptation in the lower-income region translates to escalating heatwave exposure and increased heat-stress inequality. The lowest-quartile income region is expected to experience 1.8- to 5-fold higher heatwave exposure than each higher income region from 2060 to 2069. This inequality escalates by the end of the century, with the lowest-quartile income region experiencing almost as much heatwave exposure as the three higher income regions combined from 2090 to 2099. Our results highlight the need for global investments in adaptation capabilities of low-income countries to avoid major climate-driven human disasters in the 21st century.