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Drylands represent about 41% of Earth’s land area, host more than 1,500 tree species and support more than 20% of the world’s human population. Trees are key to the functioning of numerous dryland ecosystems and contribute to goods and services for many local human communities, but many are threatened by global changes. From this perspective, mapping tree species assemblages of drylands can provide valuable information for conservation. To our knowledge, warm drylands, including hot deserts, have never been subject to a comprehensive tree biodiversity analysis independent of administrative boundaries or pre-defined regions. Our study aimed to address this gap by redefining warm drylands based on climate data and delineating bioregions using tree species assemblages at the global scale. We based the analyses on aridity and temperature data and a co-occurrence network approach using more than 1,000 tree species. Our data are mined from the Desert Trees of the World database, the Global Biodiversity Information Facility database, and the African Plant Database. This new delimitation of warm drylands reveals eight bioregions, covering about 19% of Earth’s land area across all continents. These are: North America, two bioregions in South America, the southern Mediterranean Basin and Macaronesian islands, the Saharo-Sindian region and the Horn of Africa, Southern Africa, the Socotra archipelago, and Australia. These bioregions have very distinct tree species assemblages, as well as high rates of endemism. This original diversity is found under a wide range of aridity conditions both within and between bioregions, offering the opportunity to anticipate different responses of tree assemblages face to future climate change among the world’s warm drylands. It will aid in conservation, restoration, and rehabilitation strategies involving the use of native trees among the most threatened regions worldwide.