Abstract The diamond frogs of the genus Rhombophryne are a poorly understood clade of morphologically diverse frogs native to the humid forests of Madagascar. We here present a monographic revision of the taxonomy and natural history of these frogs. We first provide morphological diagnoses and contents of the established species groups in Rhombophryne . We then integrate molecular phylogenetic data from mitochondrial and nuclear genetic markers, with morphometrics, bioacoustics, and micro-CT-based osteology, to describe seven new species from across Madagascar. This brings the number of species in the genus to 27. We have obtained DNA sequences from name-bearing specimens of 23 of these species, plus topotypical specimens of two further species; we used museomics methods to sequence name-bearing types of three species collected before 1976, as well as several other specimens of interest, allowing us to clarify taxonomic assignment of those individuals. In our treatments of all existing and new species, we provide new insights into their anatomy, diet, ecology, and distribution. We provide the first published key to the genus, which we hope will make their identification more straightforward. Finally, we suggest a revised conservation assessment in light of the data that we present. We estimate 24 of the 27 species to be threatened, with two species ( R. kilonjy sp. nov . and R. matavy ) probably worthy of being classified as ‘Critically Endangered’. Several candidate species remain taxonomically unaddressed, due to the poor representation of specimens in museum collections. We therefore emphasise the need for increased survey efforts to fill knowledge gaps in both the taxonomy and the natural history of these frogs.