The range of the genus Chiasmocleis found in the south-eastern and southern parts of the Atlantic rainforest of eastern Brazil, is expanded to the north-eastern region. Chiasmocleis alagoanus sp. nov. is described from two localities in the State of Alagoas. It is characterized by: medium size (snout–vent length 25.5–27.8 mm in females, 22.7–23.4 mm in males); snout short, tip truncate in dorsal and rounded in lateral profile; hand and foot not webbed; fingers and toes without lateral dermal spines; dorsal and ventral surfaces of body and limbs smooth, without dermal spines; colour on dorsum uniformly dark brown; venter roughly marbled in dark brown and pale cream.
