A new species (Muridae, Sigmodontinae, Phyllotini), belonging to a new genus, is described on the basis of 2 specimens from 1 locality in the mountain forests of southeastern Bolivia. Diagnostic features are posteriorly divergent edges of supraorbital region, large and hypsodont molar teeth with somewhat prismatic pattern, and anterior zygomatic process not projecting as an overhanging point. The diploid chromosome number is 56 and the fundamental number is 76. Other external, cranial, dental, and karyologic characters in the tribe and new taxon are described, illustrated, and discussed. The hypothesized phylogenetic placement of the new genus based on gene-sequence, chromosomal, and morphologic data is presented relative to evolutionary relationships of selected phyllotine taxa.