I describe a new species of reed frog (Hyperolius: Hyperoliidae) from the island of Príncipe in the Gulf of Guinea archipelago. The new species is within the Hyperolius cinnamomeoventris species complex, which includes four described species: H. cinnamomeoventris, H. veithi, H. molleri, and H. thomensis. The new species is sexually monochromatic (males and females have green dorsal coloration) and is thus distinguished from H. cinnamomeoventris, which is sexually dichromatic, and from H. veithi, in which both males and females are tan with yellow dorsolateral lines. The new species is smaller in size than H. thomensis, and differs in coloration from H. molleri and H. thomensis by lacking a black contour along the edges of the green/yellow band of coloration that extends down the dorsal side of the thigh, by lacking red/orange coloration on the dorsal and ventral sides of the thigh, and in the mitochondrial 16s and cytochrome-b genes. Furthermore, previous work indicates that the new species is distinct from H. molleri and H. thomensis in analyses of genome-wide single-nucleotide polymorphisms. This new species is the third endemic amphibian described from this small oceanic island. The type material for the closely related H. molleri and H. thomensis was destroyed or lost; therefore, I designate neotype material for these two species to enable the description of the new species.