The stump-toed frogs of the Madagascar-endemic genus Stumpffia are mostly diminutive in size, but there is one group of comparatively large frogs within the genus, which we herein refer to as the Stumpffia hara species group. Each of the four known members of this species group is endemic to a single location of deciduous dry forest with exposed karstic limestone rock. Here, we report on the discovery of members of this species group on Montagne d'Ambre, a rainforest-covered extinct volcano in the North of Madagascar that has a rich Stumpffia fauna but has been thought to lack members of the S. hara species group until now. We found two members of the species group, one at the peak, and one in transitional and dry deciduous forest on the west and northern slopes of the mountain. The high-elevation species is new to science, and we here describe it as Stumpffia bishopi sp. nov. It occupies a highly distinct position in the phylogeny of these frogs, characterized by ≥ 9.8% uncorrected pairwise distance from all other nominal Stumpffia in a fragment of the mitochondrial 16S rRNA gene. It is also the smallest of the members of the S. hara species group. Our genetic results show that the low-elevation species is Stumpffia megsoni, constituting a range expansion of that species and considerably expanding our understanding of its morphology and ecology. We report its advertisement call for the first time. Our results highlight the importance of continued surveys of even well-sampled localities, with special attention on the high elevation sites of northern massifs and collection of voucher specimens, and how much there still remains to understand about even the largest of Madagascar's small frogs.