The Peruvian Poissonia eriantha is segregated from peripatric Poissonia orbicularis and reinstated as the third unifoliolate species of Poissonia and the second from the Apurimac River basin in Peru. Poissonia eriantha is distinguished phenotypically and by DNA sequences from the ITS and cpDNA trnD-T region and morphology. This overlooked species is known from the type specimen and a recent collection from north of the Apurimac River in westcentral Cuzco where seasonally dry tropical forest vegetation predominates that is rich in succulent taxa (e.g. Cactaceae). Poissonia orbicularis is known from downstream along the Apurimac River and is disjunct further north along the Mantaro River, all within the same kind of seasonally dry vegetation. This seemingly small geographic distinction belies large genetic and phenotypic differences, a finding that may be most common to species groups confined to seasonally dry Neotropical forest vegetation. The case of Poissonia eriantha exemplifies the potentially high degree of niche conservatism and dispersal limitation that seasonally dry succulent-rich woodlands can impose on its constituent lineages. © Copyright 2011 by the American Society of Plant Taxonomists.