It is endemic to southeastern Brazil and is known from the central part of the state of Rio de Janeiro and from Serra das Torres in extreme southern Espírito Santo.

Many species in the genus Brachycephalus are bright yellow-orange, hence the name "Brazilian gold frog", but B. didactylus is all brown.

[4] "Flea-frog" or "flea-toad" was historically restricted to B. didactylus, once placed in its own genus Psyllophryne instead of Brachycephalus, but a second species, B. hermogenesi, was described in 1998 and two others, B. pulex and B. sulfuratus, have been described since then, bringing it to four species of flea-frogs/toads.

[5] At 8.6–10.2 mm (0.34–0.40 in) in snout–to–vent length,[6] B. didactylus is one of the smallest frogs in the world.

This species and the roughly similar-sized Eleutherodactylus iberia from Cuba were once regarded as the smallest,[7] but several others that are smaller have since been discovered, including the closely related B. pulex at 8–8.4 mm (0.31–0.33 in),[6] Stumpffia contumelia from Madagascar at 8–9 mm (0.31–0.35 in),[8] and Paedophryne amauensis from New Guinea at 7–8 mm (0.28–0.31 in).