We describe a new species of Rain Frog from Brazils Atlantic Forest, which harbors high richness, endemism and threatened species of anurans. Ischnocnema crassa sp. nov. is a leaf litter breeder from the montane rainforest of Santa Teresa, state of Esprito Santo, southeastern Brazil. Remarkably, the new species is so far unique amongst Ischnocnema for having moderate hand webbing between fingers. Ischnocnema crassa sp. nov. is the third species belonging to I. parva series and we present a new phylogeny based on molecular data and propose a new diagnosis for the series. We provide field and laboratory observations regarding amplexus, antipredator mechanisms, microhabitat preferences and conservation remarks. Although Santa Teresa is one of the most sampled areas in Brazil, new species are often discovered. Santa Teresa is a priority for amphibian conservation because it harbors 109 species of anurans (9% of Brazils species).