Taxonomic knowledge is essential for assessing biodiversity. When combined with species' spatial distribution, it serves as a tool for understanding ecological and evolutionary processes. In this study in the Rio de Janeiro State, Brazil, we associate the taxonomic diversity of shrub–tree Leguminosae with species locations across an island continuum with different Atlantic Forest phytophysiognomies. A total of 428 specimens, representing 61 species, were recorded and mapped. Our survey identified areas requiring increased sampling effort and highlighted priority sites for conservation, based on diversity, taxonomic representation, and the presence of threatened species. The highest species richness and specimen abundance were concentrated below 500 m a.s.l., within Lowland and Submontane Dense Ombrophilous Forests. Notably, only four species were represented by more than 20 specimens, while nearly 50% of species had just two specimens in the island's vegetation sampling.