We describe a new glassfrog belonging to the genus Hyalinobatrachium from the tropical wet forests and premontane rainforests of the Caribbean foothills of Costa Rica, at elevations between 400-800 m. The type locality is approximately 4km west of Santa Clara, ca. 400 m a.s.l. (N 10.219, W 83.949). This new taxon is distinguished by its morphological characteristics, distinct advertisement call, and genetic distance (COI mRNA). The new species can be distinguished from other species of the genus Hyalinobatrachium by the combination of the following characters: (1) snout truncate in dorsal and lateral views; (2) granular dorsal skin; (3) parietal and cardial peritonea transparent; (4) hand webbing formula III 2(-)-2(+) IV; (5) in life having a uniform lime green dorsal surface that lacks any evident light or dark spots; (6) iris coloration silvery-white with fine dark spots or reticulation; (7) advertisement call consisting of a single tonal long metallic whistle-like note with a duration of 0.40-0.55 s (average 0.501 s) and a dominant frequency of 3.35-3.44 kHz (average 3.39 kHz); (8) highly divergent DNA sequences for the mitochondrial gene COI from other analysed members of the genus Hyalinobatrachium, with 12.4% divergence from its closest known species, Hyalinobatrachium chirripoi.