Rulyrana erminea is an endemic glass frog from Peru associated to rainforests, bamboo forests, streams and converted areas. Previously, the species was known from a single female specimen. Herein, I report four additional specimens from three new localities, among them the first male specimens and the first record for the Department of Cusco, the new southernmost record for the species, extending its distribution 111.5 km airline to the south. The species is recorded between 370 - 870 m a.s.l. One specimen was found inside the Machiguenga Communal Reserve.