Piper globirhachis, a new species from northwestern Amazon, is described and illustrated. It occurs across the Marañon River Valley from its close relative P. nudilimbum C. DC. with which it has been confounded for ca. 100 years. Piper globirhachis differs morphologically from P. nudilimbum in having ovate to long–ovate leaf blades, and globose inflorescences, vs. elliptic leaves, and elongate inflorescences in P. nudilimbum. These species also differ in geographic distribution and ecological niche. A molecular phylogeny based on 135 ITS sequences (125 from Neotropical Piper species and 5 Asian Piper species) enabled the species boundary test (Poisson-tree-processes, PTP) corroborating that these are distinct species. Furthermore, these two species formed a well-supported clade sister to subgenus Oxodium (=Schilleria). This phylogeny highlights the importance of allopatric speciation in Piper and provides evidence for the resurrection of Pleiostachyopiper as a subgenus of Neotropical Piper. An identification key for the subgenera of Neotropical Piper is provided

We extracted DNA from herbarium specimens using the DNAeasy plant mini kit (Qiagen, Valencia, California). The ITS region was amplified using one of two pairs of primers ITS5-ITS4, or LEU1-ITS4 (Baldwin 1992). Sequencing was contracted with GenCore (Universidad de los Andes, Bogotá, Colombia). The resulting sequences were manually aligned against previously obtained alignments (Jaramillo et al. 2008). We selected 135 ITS sequences from the large alignment to portray here. Five sequences from Piper species from Asia and the South Pacific were used as outgroups. A total of 125 species (130 sequences) of Neotropical Piper, comprising representatives of all major clades, were used to identify the relationships of P. nudilimbum and the new segregate.