Globally, tropical dry forests are rapidly disappearing due to human activities. In northwestern Madagascar, there are rare examples of primary dry forest stands even though most dry forests in the nation have been lost. To understand the structure and tree species diversity of a primary stand of dry forest in Ankarafantsika National Park, we established a 15-ha monitoring plot. We mapped all woody stems with diameter at breast height (DBH) ≥ 5 cm. We identified 35,757 tree individuals, which encompassed 140 species belonging to 44 families. The size structure was strongly biased to small sizes. The mean number of individuals per ha was 1771, 534, 68, and 10 for trees with DBH ranges of 5-10 cm, 10-20 cm, 20-30 cm, and ≥ 30 cm, respectively. The two most abundant species in terms of stem number accounted for 19.3% and 13.1% of all individuals. Our study is the first that revealed the structure of a primary dry forest in Madagascar, which supports regional biodiversity and provides multiple ecosystem services. Given rapid degradation and loss of dry forests around the world, it will be important to build a regional and global network of research and conservation of tropical dry forests.