Healthy aquatic ecosystems are vital for ensuring water security, yet they face significant pressures from human activities. In many parts of Asia and the Pacific, these pressures are so severe that water security is increasingly threatened by environmental degradation alone. Unhealthy aquatic ecosystems and catchments diminish the reliability of water supply and elevate the costs of water treatment, particularly for drinking purposes. Balancing human water needs with the preservation of ecosystem services is one of the most pressing challenges of the 21st century and is crucial for achieving the Sustainable Development Goals, with water security at its core. As part of the Asian Water Development Outlook, a flagship publication of the Asian Development Bank, we have developed a methodology to assess the health of aquatic ecosystems on a global scale and its potential impact on a country’s water security. Given the challenges of quantifying aquatic ecosystem health at broad spatial scales, we use spatially distributed indicators that are key causal factors influencing ecosystem health. These indicators are derived from various sources, including remote sensing and global models, with several relying on long-term trend analyses. The indicators include groundwater depth, riparian vegetation loss, river flow alterations, riverine connectivity, and water quality pressures. Our analysis reveals that countries with poor aquatic ecosystem health typically experience lower water quality, higher levels of flow alteration, and reduced riverine connectivity. In contrast, countries with healthier ecosystems tend to be earlier in their development pathways and have lower pressures from these drivers. These findings highlight the significant challenges of maintaining healthy aquatic ecosystems while securing water resources. They also underscore the importance of addressing specific drivers of ecosystem degradation, which individual countries can target to improve both ecosystem health and water security. By doing so, nations can enhance their resilience and meet the growing demand for water while safeguarding the environment.