We describe a new species of marsupial frog (genus Gastrotheca) from an isolated patch of cloud forest in the upper reaches of the Pachachaca River, a tributary of the Apurímac River in southern Peru (Apurímac Region). The new species is small with males less than 30 mm and a single female 35.3 mm in snout-vent length. A juvenile was collected from inside an epiphytic bromeliad at ∼3 m, suggesting that this species is arboreal or uses bromeliad tanks as microhabitats. We describe the advertisement call, which has a dominant frequency above 2.2 kHz. Based on morphology and advertisement call, the new species is most similar to Gastrotheca piperata described from the Yungas of Bolivia. The new species differs from G. piperata by having smaller size, no dermal fold on the tarsus, a bluntly rounded to vertical snout in profile, gray coloration on dorsal surfaces of Fingers I and II and by producing advertisement calls that have a higher dominant frequency and a smaller number of pulses. © 2011 Society for the Study of Amphibians and Reptiles.