From the 259 species of legumes recorded, the distribution of 242 wild species of legumes over 224 locations within 12 plant communities was examined in northeastern Mexico. Objectives were: (1) to determine diversity of legumes in the mountains and plains of northeastern Mexico and (2) to elucidate distributional patterns of legumes in this region. The subfamily Papilionoideae had the largest number of wild genera (47) and species (167), followed by Mimosoideae (13 and 44) and Caesalpinioideae (10 and 31). Genera with the largest number of native species were Dalea (28), Desmodium (16), Astragalus (13), Senna (13), Acacia (11), Phaseolus (10), Crotalaria (9), and Lupinus (8). Of wild legumes, 24 genera had ≥3 species each and 21 species were endemic to this area; most were Lupinus (5 species), Astragalus (4), and Dalea (4). Almost all of the 21 endemic species were >1,500 m in elevation in oak-pine forests (7), oak forests (5), and coniferous forests (5). Only one endemic species occurred <1,400 m in elevation. Of the endemic species, 90.5% were in the subfamily Papilionoideae. There were 17 cultivated legumes, most of them in Caesalpinioideae. Similarity and dissimilarity matrixes using the Sørensen index were assessed using minimum-variance clustering. Using diversity of legumes, three assemblages of plant communities were recognized. Oak, oak-pine, mesic-conifer, and cool-temperate coniferous forests harbored the highest diversity of legumes, while rosetophyllous and xeric scrublands and halophytic communities contained the lowest diversity.