We describe a new species of high-elevation rain-forest tree frog (genus Platymantis) from New Britain in the Bismarck Archipelago, Papua New Guinea. It is characterized by moderate body size (males 27.4–30.7 mm), widely expanded finger and toe disks, smooth dorsal skin, a distinct reticulate dorsal color pattern, and numerous spectral and temporal characteristics of the advertisement call. The new species inhabits shrub layer vegetation in canopy gaps in primary montane rain forests of the Nakanai Mountain Range above 1500 m. We compare the new species to all known Platymantis from New Britain and to other morphologically similar species from the Bismarck archipelago, Solomon Islands, and Fiji. We also redescribe Platymantis macrosceles from the holotype and two new specimens from the Nakanai Mountains. We suspect that anuran species diversity on the large and topographically complex island of New Britain is currently underestimated.