Widespread heat stress will become the norm in a warming Southeast Asia.

Southeast Asia faces growing risks from extreme heat under climate change, yet existing assessments do not fully account for physiological impacts and population-level exposure, particularly at resolutions relevant for planning. Here we present a comprehensive evaluation of future heat stress in the region by integrating three methodological innovations: physiologically relevant bio-climatic indices, spatially explicit population exposure, and sub-daily temporal resolution. Using the Universal Thermal Climate Index (UTCI) and Wet-Bulb Globe Temperature (WBGT), we assess heat stress across Southeast Asia at [Formula: see text] km spatial and 3-hourly temporal resolution under low- and high-emissions scenarios. We combine these projections with population data to identify where and when risks are most acute. Our results show that even by the near-future (2030–2059), exposure to life-threatening extremes (UTCI > [Formula: see text]C, WBGT > [Formula: see text]C) increases sharply, by factors of 2.8–4.6 (UTCI) and 4.6–7.9 (WBGT) relative to historical levels. The number of people exposed to at least one consecutive week of extreme UTCI grows from 9 million historically to 23–28 million under RCP2.6 and RCP8.5, while extreme WBGT exposure increases from 0.1 million to 7–17 million. Continental Southeast Asia, including Myanmar, Thailand, and Cambodia, faces the most acute risks, with 6–9 hours of severe heat stress per day during peak months. By the far-future (2070–2099) under RCP8.5, exposure escalates to catastrophic levels, with up to 200 days per year of unsafe conditions and exposed populations increasing more than tenfold. Our findings show that dangerous levels of heat stress will emerge within decades, underscoring the urgency of adaptation and the benefits of strong mitigation.