Marine heatwaves (MHWs) are becoming more frequent and intense, significantly impacting global marine ecosystems. The study quantifies trends and attributes of MHWs across all ocean basins and examines their biological impacts from species to ecosystems. MHWs are particularly vulnerable in regions with high biodiversity, species found at their warm range edges, or concurrent non-climatic human impacts. Physical attributes of prominent MHWs vary, but all have detrimental effects on various biological processes and taxa, including critical foundation species like corals, seagrasses, and kelps. MHWs are emerging as powerful agents of disturbance, capable of restructuring entire ecosystems and disrupting ecological goods and services in the coming decades. The study uses a meta-analysis to examine ecological responses to eight prominent MHW events, finding overall negative effects on biota across research papers, events, taxa, and response variables. Populations near the warm-water limit of species' distributions tend to respond more negatively to MHWs, indicating that they are particularly vulnerable. The increased frequency and duration of MHWs pose a significant challenge for coastal societies, highlighting the need for effective management and mitigation strategies to protect marine ecosystems.Marine heatwaves (MHWs) are becoming more frequent and intense, significantly impacting global marine ecosystems. The study quantifies trends and attributes of MHWs across all ocean basins and examines their biological impacts from species to ecosystems. MHWs are particularly vulnerable in regions with high biodiversity, species found at their warm range edges, or concurrent non-climatic human impacts. Physical attributes of prominent MHWs vary, but all have detrimental effects on various biological processes and taxa, including critical foundation species like corals, seagrasses, and kelps. MHWs are emerging as powerful agents of disturbance, capable of restructuring entire ecosystems and disrupting ecological goods and services in the coming decades. The study uses a meta-analysis to examine ecological responses to eight prominent MHW events, finding overall negative effects on biota across research papers, events, taxa, and response variables. Populations near the warm-water limit of species' distributions tend to respond more negatively to MHWs, indicating that they are particularly vulnerable. The increased frequency and duration of MHWs pose a significant challenge for coastal societies, highlighting the need for effective management and mitigation strategies to protect marine ecosystems.