RESUMO: Cleidella Roza & Mermudes, 2020 is a small genus of Mastinocerinae currently known only from montane areas of the Atlantic Rainforest of Rio de Janeiro State, Brazil. The genus comprises two species, C. picea Roza & Mermudes, 2020 and C. silveirai Roza & Mermudes, 2020, recorded from Parque Nacional da Serra dos Órgãos and Parque Estadual da Ilha Grande. Here, we describe a third species, C. correcta sp. nov., from the Parque Estadual do Desengano, northern Rio de Janeiro State. The new species resembles C. picea in overall coloration but differs mainly by the size and shape of the eyes, and length of antennomeres. We provide a diagnosis, description, illustrations and an updated identification key to the species of Cleidella. The discovery of C. correcta reinforces the importance of Atlantic Rainforest montane areas as centers of endemism of soft-bodied beetles, like Phengodidae, and highlights the still poorly known diversity of Mastinocerinae in southeastern Brazil.