We name and describe a new species of long-tailed sigmodontine rodent of the genus Tanyuromys, based on 3 specimens collected from 3 different localities in northern Ecuador, during the period 1953–2008. All 3 localities are at middle elevations on the Pacific Andean slopes and specimens were taken both in intact and disturbed forest areas. Tanyuromys previously has been treated as including a single species—T. aphrastus (Harris, 1932); however, using morphological and molecular characters (mitochondrial cytochrome b [Cytb], 1,143 bp), we herein recognize 2 species: T. aphrastus proper, which, so far as known, occurs only in Costa Rica and Panama, and a new species known only from Ecuador. The Ecuadoran species, like T. aphrastus, is characterized by a long tail (ca. 1.5 × length of head plus body); large, complexly constructed, pentalophodont molars; braincase with beaded supraorbital margins continuous with raised temporal ridges; short, anteriorly constricted incisive foramina; short, narrow palatines; and small auditory bullae. Although similar morphologically, which explains why the 2 species have hitherto been regarded as 1, the few specimens available of each differ in that the new species has, among other traits, a narrower interorbital breadth, narrower zygomatic plate, invariably (thus far) distinct supraorbital bead continuing posteriorly over parietal to lambdoidal crests as a prominent raised ridge, relatively large lateral wing of the parietal, skull with more angular rather than more rounded contours. Molecular and morphological characters confirm that specimens of T. aphrastus from Costa Rica and Panama are quite similar; they differ in Cytb sequences by 1.2%. The new species from Ecuador, in contrast, differs from the Central American T. aphrastus in Cytb sequences by 10.2%. The 2 species form a well-supported clade; we infer the genus' phylogenetic associations within the Oryzomyini, based on the complete Cytb sequence.