Thomasomys pagaibambensis is a species of rodent in the genus Thomasomys known from the Andes of northern Peru. It consists of populations that were identified as Thomasomys cinereus until they were identified as a new species in 2023.
The species occurs in montane forests of the Pagaibamba Protection Forest in northern Peru, at an elevation of . The scientific name refers to the name Pagaibamba. Three other species of Thomasomys are known from the area, T. taczanowskii, T. pyrrhonotus, and a species tentatively identified as T. aureus. The animal eats insects and seeds.
Thomasomys pagaibambensis is medium in size for its genus, with a head-body length of . Characteristic traits include brownish fur, a long, one-colored tail with a white tip, and relatively short incisive foramina, an opening in the palate.