The study examines the impacts of marine heatwaves (MHWs) on critical, habitat-forming foundation species (macroalgae, seagrass, corals) in 1322 shallow coastal areas across 85 marine ecoregions. Intense summer MHWs are found to significantly contribute to the decline of these foundation species globally, with detrimental effects increasing towards species warm-range edges and over time. The study identifies several ecoregions where foundation species do not respond to MHWs, suggesting some resilience to warming events. Key factors mediating impacts include cumulative MHW intensity, absolute temperature, and location within a species' range. The findings highlight the potential for many coastal ecosystems to lose foundation species, impacting biodiversity, ecological function, and ecosystem services. Understanding the relationships between MHWs and foundation species can help predict impacts and develop management and adaptation strategies. The study also discusses the increasing frequency and intensity of MHWs, their global impacts, and the need for further research to understand compound stressors and recovery processes.The study examines the impacts of marine heatwaves (MHWs) on critical, habitat-forming foundation species (macroalgae, seagrass, corals) in 1322 shallow coastal areas across 85 marine ecoregions. Intense summer MHWs are found to significantly contribute to the decline of these foundation species globally, with detrimental effects increasing towards species warm-range edges and over time. The study identifies several ecoregions where foundation species do not respond to MHWs, suggesting some resilience to warming events. Key factors mediating impacts include cumulative MHW intensity, absolute temperature, and location within a species' range. The findings highlight the potential for many coastal ecosystems to lose foundation species, impacting biodiversity, ecological function, and ecosystem services. Understanding the relationships between MHWs and foundation species can help predict impacts and develop management and adaptation strategies. The study also discusses the increasing frequency and intensity of MHWs, their global impacts, and the need for further research to understand compound stressors and recovery processes.