We describe a new species of Cettia from the Crown Prince Range, Bougainville Island, North Solomons Province, Papua New Guinea. By combining morphometric and molecular phylogenetic techniques, we attempt to broaden our understanding of evolutionary processes within the genus Cettia in the southwest Pacific. The new species proves to be distinct with respect to several morphological characteristics that are most probably related to a more terrestrial lifestyle than that of its congeners. Molecular data agree with morphological data in establishing that these birds are at least as distinct from the other island forms of Cettia as those forms are from each other, far exceeding intraspecific variation. These data and the restricted distribution of the population on Bougainville strongly support recognition of a new species. The application of molecular phylogenetic techniques also supports the idea that the new species and other island forms of Cettia confined to mountains on southwest Pacific islands are derived from a single common ancestor rather than being independently derived from one or more mainland forms. In addition, the relatively recent discovery of two new species of Cettia suggests that additional forms await discovery in other poorly known areas of the southwest Pacific. Our results point to the need for further molecular studies and for additional field research into the distribution and ecology of forest songbirds on islands.