A new species of squeaker frog (Arthroleptis) is described from Ngozi Crater in the Poroto Mountains of southwestern Tanzania. The holotype, and only specimen, was collected 80 years ago but remained undescribed because of past taxonomic confusion relating to large-bodied species of Arthroleptis from eastern Africa. Morphological study, including multivariate analysis of measurement data, indicates that this new species is distinguishable from other species of Arthroleptis, including large-bodied species such as Arthroleptis adolfifriederici, Arthroleptis affinis, Arthroleptis nikeae, Arthroleptis nguruensis, and Arthroleptis tanneri. The new species differs from other described species of Arthroleptis by the combination of its large body size, relatively long toes, and relatively small tympanum. The description of this new species further highlights the biological diversity of the Southern Rift Mountains.