In 2024, global temperatures reached record highs multiple times, highlighting the growing severity of extreme heat and the widespread implications. By 2050, Bangkok, already prone to high temperatures, can see extreme heat risks to health, productivity, economic growth, and infrastructure. The Urban Heat Island (UHI) effect exacerbates these challenges, making some districts, including those with high concentrations of vulnerable populations, significantly hotter than surrounding rural areas. This report, developed by the Bangkok Metropolitan Administration (BMA), in partnership with the World Bank, examines the growing threat of extreme urban heat in Bangkok, long-term trajectories, and potential impacts. It also quantifies the health, economic, and infrastructure costs associated with rising temperatures and explores targeted interventions and strategic reforms - particularly for the city's most vulnerable communities.