The rainforests of the Marojejy massif in northern Madagascar are a well-known hotspot of amphibian species diversity and endemism. In the present paper, we re-describe Rhombophryne serratopalpebrosa (Guibé 1975), a cophyline microhylid frog from high altitude on this massif, based on a re-examination of its holotype, and describe Rhombophryne vaventy sp. nov. using characters of external morphology and osteology, illustrated by pdf-embedded comparative 3D models of their skeletons. Rhombophryne serratopalpebrosa differs from R. vaventy sp. nov. by smaller size (28 mm snout-vent length vs. 52.9 mm), skin texture (granular vs. rough and tubercular skin respectively), supratympanic fold shape (strong, long and straight reaching the eye vs. curved and not extending anteriorly beyond the tympanum), relative tympanum diameter (78% vs. 41% of eye diameter), shape of the postchoanal prevomerine palate, shape of the footplate of the columella, length of prepollex, and by other subtle osteological features. Morphological comparisons suggest that a specimen from Ambolokopatrika assigned to R. serratopalpebrosa in previous genetic studies might belong to yet another undescribed species, closely related to R. vaventy sp. nov., whereas DNA sequences of the topotypic R. serratopalpebrosa remain unknown. We therefore emphasise the need for collecting additional material from high altitudes of the Marojejy massif to understand the systematics, as well as the natural history, of this poorly known species. For the new species described herein, we propose a Red List threat status of Vulnerable, in line with other Marojejy endemics from a similar altitude.