We describe two new species of forest frogs in the genus Platymantis from the Isabel Island group, Solomon Islands. One new species is a medium-sized, terrestrial form that is morphologically most similar to P. weberi (a widespread Solomon Islands species). The other new species is an arboreal frog that is morphologically similar to Platymantis neckeri (known from Bougainville, Choiseul, and Isabel islands). Both new species possess unique advertisement calls that distinguish them from all sympatric congeners. Because acoustic characteristics function as the primary mate-recognition signals for anuran species, and are therefore an excellent indicator of the status of unique evolutionary lineages, we recognize each as new species. We diagnose both new species on the basis of their distinctive advertisement calls and in the case of the terrestrial form, by differences in body size, body proportions and skin texture. The diversity of ceratobatrachid frogs of the Solomon islands and Bougainville is underestimated and in need of a comprehensive taxonomic review coupled with a standardized survey of acoustic characters.