In this report, the fifth and last of our monographic series on mammalian diversity and ethnomammalogy in the Yavarí-Ucayali interfluvial region of northeastern Peru, we document the local occurrence of 40 species of rodents, including 5 sciurids, 17 cricetids, 1 caviid, 1 cuniculid, 2 dasyproctids, 1 dinomyid, 2 erethizontids, and 11 echimyids. The following substantive taxonomic results, among others, are reported: (1) We discuss current issues of sciurid classification and treat all New World tree squirrels (Sciurini), except North American Tamiasciurus, as members of the genus Sciurus; the proposed subgeneric classification is monophyletic, and it conserves longstanding binomial usage for most species. (2) We describe a new species of squirrel, Sciurus (Hadrosciurus) pachecoi, which had previously been identified as a distinct lineage by molecular analyses.