We describe a new species of high-elevation rainforest tree frog (genus Platymantis) from the Nakanai Mountains, New Britain Island, Bismarck Archipelago, Southwestern Pacific. The new species is characterized by moderate body size (34.2–35.8 mm for four males), widely expanded terminal digital disks of the fingers and toes, smooth skin of the dorsum, a distinctive color pattern, and a complex, amplitude-modulated advertisement call produced in groups of 3–6 notes. We compare the new species to all known species of Platymantis from New Britain and to additional phenotypically similar species from the Solomon Islands and Fiji. It is most similar to P. nexipus, a species known previously from only a single specimen. We rediagnose and redescribe P. nexipus on the basis of the holotype and ten recently collected specimens, provide the first descriptions of the advertisement calls of both species, and comment on an additional suspected undescribed species from the Nakanai Mountains of New Britain Island.