Macrostemum Kolenati is the largest genus of Macronematinae, with about 106 described species, being 10 described females and only three had their female genitalia illustrated and described: Macrostemum araca Pes, Desidério, Barcelos-Silva & Hamada, 2019, M. brasiliense (Fisher, 1970) and M. scharfi Pereira & Calor, 2020. For the other seven species with described females, only the wings are illustrated. Herein, a new species of Macrostemum is discovered from the Serra da Mocidade, an isolated mountain range located in Roraima State, in the northern Brazilian Amazon. Macrostemum mocidade sp. nov. is described and illustrated based on males and females. It can be recognized as the adult male mainly by the morphology of the tergum X and phallic apparatus. Interestingly, the female of the new species have a very unusual genitalia compared to the other species in the genus, being very tapered, with a pair of cerci and two pairs of papillae very long and slender.