We undertook a phylogenetic study of the genus Piptadenia with a sample of 18 of ca. 27 recognized species using DNA sequences from the plastid Trnl-F and trnK/matK regions. Parsimony jackknife results support the hypothesis that Piptadenia is polyphyletic and that there are three independent lineages. The largest groups of species form a monophyletic lineage that is sister to Mimosa and corresponds closely to Piptadenia section Piptadenia sensu Barneby. Three species (P. obliqua, P. moniliformis, and P. leucoxylon) of section Pityrocarpa are sister to species of Pseudopiptadenia in a lineage with Parapiptadenia, Microlobius and Stryphnodendron. Piptadenia viridiflora is distantly related to sects. Piptadenia and Pityrocarpa. Section Piptadenia is characterized by the presence of prickles (aculei), a potential synapomorphy linking it to Mimosa, and exserted, racemose or paniculiform inflorescences of aggregated spikes. Piptadenia viridiflora has stipular spines and 1–2 axillary, spicate primary inflorescences. Species of sect. Pityrocarpa are unarmed trees (sometimes with hardened, persistent stipules) whose flowers have recurved petals and an ovary on a long gynophore. We recommend that Piptadenia viridiflora be removed to a new genus, and that Piptadenia be restricted to the other species of sect. Piptadenia. We resurrect the generic name Pityrocarpa to encompass the Pityrocarpa clade and make the following new combinations: Pityrocarpa moniliformis, Pityrocarpa obliqua ssp. brasiliensis, and Pityrocarpa leucoxylon.