We describe a new species of Gastrotheca from the Ro Abiseo National Park of the San Martn Region in the Andes of northern Peru. The description is based on a series of 29 specimens that were collected between 1989 and 1999 at two localities (Pampa del Cuy, Alpamachay) in the wet puna of the national park between 3380 and 3470 m elevation. The new species has a snoutvent length of 46.957.7 mm (53.5 ± 3.0) in females (n  =   21), and 35.343.8 mm (41.6 ± 3.2) in males (n  =  6). In life, the dorsum is pale grayish brown with reddish-brown paravertebral markings or blotches, and the venter is cream. Females have a single median, dorsal brood pouch and release tadpoles in water as indicated by two specimens found at Gosner Stages 45 and 46. The new species is distinguished from all of its congeners by having the dorsum with two reddish-brown paravertebral markings bearing prominent longitudinal ridges and warts. The new species differs from the two other species of Gastrotheca (G. ossilaginis, G. phalarosa) known from the Region San Martn by lacking integumentary-cranial co-ossification (present in G. ossilaginis), by having the skin on dorsum with prominent longitudinal ridges and warts (dorsal skin shagreen in G. ossilaginis), and in producing tadpoles (direct development in G. ossilaginis). The new species most closely resembles G. phalarosa. Both species produce tadpoles, have fingers and toes with lateral fringes, and Finger I larger than Finger II. Gastrotheca phelloderma differs from G. phalarosa by having the skin on dorsum with prominent longitudinal ridges (absent in G. phalarosa), the venter granular (smooth except granular on posterior part of belly and proximal posteroventral surfaces of thighs in G. phalarosa), a palmar tubercle (absent in G. phalarosa), a long and distinct inner tarsal fold (barely evident on distal fifth of tarsus in G. phalarosa), and the throat and belly grayish tan (black with white spots in G. phalarosa). © 2011 The Herpetologists' League, Inc.