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With approximately 30 species, Mimosa ser. Leiocarpae is the largest series of M. sect. Batocaulon and is particularly abundant in semi-arid environments of South America. Previous studies have shown that the pollen type is remarkably congruent with the plastid gene tree and it presents relatively low levels of homoplasy, having utility for phylogenetic reconstruction within Mimosa. Pollen morphology of 29 of the 30 species belonging to Leiocarpae was studied using scanning electron microscopy to identify diagnostic characters and to test taxonomic and phylogenetic hypotheses. The pollen grains of Leiocarpae are dispersed in tetrads or bitetrads, which are decussate, isopolar or heteropolar, compressed or not, elliptical or ellipsoidal outline. The apertures are porate. The ornamentation is areolate-verrucate, and the size of grains varies from 8.5 to 43.7 m in the longer and 7.1 to 20 m in the shorter dimension. The pollen morphology does not support the monophyly of Leiocarpae, which is in accordance with evidence from molecular phylogenetic studies.