The green belt surrounding S??o Paulo and Santos metropolises, designated by UNESCO as the S??o Paulo City Green Belt Biosphere Reserve - GBBR (1994), encompasses a population of 25,4 million people in a region responsible for nearly 20% of Brazilian GDP. With approximately 2,33 million hectares, the biosphere reserve and its inner urban area (220 thousand hectares) are host to expressive terrestrial and marine biodiversity, typical to the Atlantic Forest and Cerrado biomes. This paper presents the biosphere reserve as a water security territory: it provides around 90% and 100% of the water flows supplying S??o Paulo and Santos metropolises, respectively. Representing only 7,36% of S??o Paulo State territory, the GBBR harbors over 70% of the state??s volume for public supply. Spatial analysis shows that most of this volume comes from a few protected areas that act as hydric hotspots. During 2013-2015 Southeastern Brazil and especially S??o Paulo faced a severe shortage in rainfall that posed threats to the metropolitan water supply, leading to important reflections about the region??s hydric vulnerability. Combining classical built infrastructure with ecological infrastructure (conservation, restoration and sustainable use of its ecosystems) is paramount to ensure the population??s water security in the medium and long terms. In this sense, the geographical and conceptual frameworks constituted by the GBBR can provide integrated land management solutions for improving water supply in quantity and quality for such a complex and critical territory.