Caviomorphs  are  the  neotropical  hystricognathous  rodents  (Ctenohystrica:  Hystricognathi: Caviomorpha). Octodontoidea are today the richest and most speciose clade of  caviomorphs.  Their  fossil  record  also  reveals  a  wide  array  of  extinct  species,  including  Acarechimys, a taxon with a widespread South American distribution and extensive temporal range. This genus comprises at least five species documented in deposits ranging from the late Oligocene  to  the  Middle  Miocene.  Here,  we  report  the  discovery  of  a  new  species  of  Acarechimys from Upper Miocene deposits of the Envira River (PRE06 outcrop), situated in the Acre State, western Brazilian Amazonia. The new species differs from the other species of the genus in having lower molars with a strong posterior arm of the metaconid, the presence of a mesostylid (usually prominent) and a neomesolophid forwardly directed and connected to the metalophulid I on m1 but not on m2–3, and the absence of posterior arm of the protoconid. This  taxon  represents  the  youngest  known  species  of  the  Acarechimys  lineage  to  date.  Its  occurrence in Western Amazonia strengthens support for the hypothesis that South American rodent  lineages  survived  longer  during  the  Neogene  at  low  latitudes  than  in  the  southern  regions of the continent.