Amphibians of the Eastern Arc Mountains are poorly known, apart from those of the East Usambara, Uluguru, and Udzungwa Mountains. Here, we detail specimens collected in the northwestern Eastern Arc Mountains (West Usambara, South Pare, and North Pare) over the past 100 years. Although not comprehensive, the list is compiled from specimens held at institutions in Europe and the United States and in personal collections; it includes most of the specimens known from this area. From these specimens, we describe one new species of the frog genus Arthroleptis and discuss other specimens that may represent new species. We add substantially to the known diversity in these areas. Using the compiled species list, we assess species diversity and relate the differences to sampling bias and possible biogeographic differences among the mountains. The future preservation of amphibians across this area is of concern given the increasing loss of forest.