We describe a new frog species of the genus Guibemantis in the subgenus Pandanusicola from southeastern Madagascar and provide a redescription of a morphologically similar species, G. pulcher, based on its type series and on newly collected material. The new species, Guibemantis tasifotsy, differs from most Pandanusicola by probably not breeding in leaf axils of Pandanus plants but instead was collected calling from the vegetation above large, open lowland swamps. The green dorsal color of the new species is reminiscent of Guibemantis pulcher but can be distinguished from this Pandanus-breeding species by the presence of prominent white blotches along the flanks and by the structure of femoral glands in males. It shares these character states with G. liber from which the new species differs in details of coloration and advertisement call. Newly determined DNA sequences of the cytochrome b gene confirm that G. tasifotsy is genetically highly divergent from all other species of Guibemantis.