<p>Key factors, such as climate change and changes in land use and cover, amplify stress on water resources, making them vulnerable. The need to ensure these resources in sufficient quantity and quality to sustain their multiple uses highlights the relevance of determining their natural vulnerability to reverse extreme scenarios. Within the boundaries of a watershed, its assessment can contribute to addressing the challenges of the region’s water security. In this context, this study aims to evaluate the natural water vulnerability in three major watersheds in Brazil, which are essential for supplying water for human consumption and industrial activities: the Doce, Paraíba do Sul, and Tietê river basins. The methodology employed involved the development of a Natural Water Vulnerability Index (NWVI), conceived from a multicriteria decision analysis approach using the Fuzzy AHP method. This index was integrated into Geographic Information Systems (GIS), allowing spatial analysis through five critical environmental factors: precipitation, land use and cover, elevation, slope, and soil texture. The results showed that precipitation is the most relevant factor, followed by land use and cover. The analysis revealed an increase in vulnerability in areas near the outlets of the basins. The Tietê river basin proved to be the most vulnerable, while the Doce river presented many areas with very high levels of vulnerability. This information indicates the need for intensive management strategies in the most vulnerable areas and the importance of continuously monitoring the less vulnerable areas to prevent future increases in vulnerability.</p>