During a faunal survey in the foothills of the Ecuadoran Andes southwest of the Cordillera del Cóndor, a mouse of uncertain affinities was taken in a fishing net. Various external characters suggested that it was a member of the genus Rhagomys, previously unrecorded in Ecuador. Comparisons with the external, cranial, and dental morphology both of Rhagomys rufescens and R. longilingua identified a number of unique characters, including its long, narrow rostrum and incisive foramina and the reduced anterolingual conule on its M1 procingulum. We describe the mouse as a new species of Rhagomys; both its morphology and molecular analyses suggest that it is sister to R. longilingua. This record of Rhagomys in southern Ecuador extends the known distribution of the genus 700 km northward and adds yet another genus and species to Ecuador's extensive list of rodent species.