We show that heatwave exposure has disproportionately increased in the lowest-income regions globally compared to the highest-income regions over the past four decades. We also show that emerging heat hazards (e.g., shock heatwaves – the first heatwave of the season – and widespread contiguous extreme heat events) intensified in the past four decades and are expected to increase in the future across the globe, jeopardizing adaptation efforts even in wealthy countries. We use climate projections to evaluate future changes in heat-stress inequality globally. In doing so, we incorporate the fact that high-income countries have greater institutional and individual capacity to rapidly adapt to climate change than low-income countries. Our findings demonstrate continued increases in heatwave exposure inequality because of delays in adaptation capacity in the developing world, compounded by a higher emergence of warming in low-latitude areas where most of the low-income countries occur. These results highlight the urgency to scale adaptation efforts in the low-income regions to minimize heat-stress inequality.