Toadlets of the genus Brachycephalus are endemic to the Brazilian Atlantic Rainforest, where they inhabit the forest floor leaf litter. These miniaturized frogs are commonly known as flea-toads or pumpkin toadlets depending on their phenotypic characteristics. Herein, we describe a new species of the genus Brachycephalus from montane forests (970–1200 m a.s.l.) in the Serra dos Órgãos mountain range, in the state of Rio de Janeiro, southeastern Brazil. The new species is characterized by having a small body size within the genus (maximum SVL 9.6 mm in males and 10.9 mm in females), a noticeably granular dorsum with an X-shaped mark and a median row of small yellow tubercles, a yellow background color with red blotches on ventral surfaces, a lateral dark brown stripe, and the absence of hyperossification in the skeleton. Morphological features of the new taxon resemble those of both flea-toads and pumpkin toadlets. Based on our molecular phylogenetic hypothesis, the new species is recovered sister to the clade formed by the monophyletic B. ephippium and B. vertebralis species groups (previously considered as lineages of a more inclusive B. ephippium group). We also reinforce the suggestion of not using the name B. didactylus group because it does not represent a monophyletic taxon. The new species probably has an extremely restricted geographic distribution, and its discovery reinforces the importance of the Serra dos Órgãos mountains for the conservation of the Atlantic Rainforest anuran biodiversity.