The spiny-rats of the genus Proechimys are the most speciose and widely distributed of the Echimyidae. Within them, P. simonsi is one of the largest and perhaps one of the most readily recognizable due primarily to the white coloration ventral fur, fine and soft hairs, hypothenar pad on plantar surface of hindfeet absent, and the deep mesopterygoid fossa. Its distribution comprises lowland rainforest and montane forests (yungas) on the eastern slope of the Andes, with elevation range upward to 2000 m. In a recent assessment to the northern Peruvian yungas, Amazonas department, Bongará province, a spiny-rat of the genus Proechimys was captured at 2439 m, an elevation very unusual for the genus. This specimen was compared with P. simonsi from the type region in Junín and from other populations, and found that it is morphologically distinct. This specimen is externally similar to P. simonsi, but it is comparatively smaller and darker, the fur is longer and softest, the skull is slimmer, the ventral fur and the metatarsals are immaculately white, the fourth upper molar is smaller than the third, and the tail length is approximately 90% of head and body length. Based on these differences, we suggest that this specimen represents a new species of the simonsi group. Our statement was complemented with a phylogenetic analysis of the mitochondrial DNA sequences, which showed the Bongará’s specimen has a genetic distance of 9 to 13 % compared to other populations. This analysis also corroborates that P. simonsi represents a species complex.