Historically, specimens representing the Peromyscus boylii species group from west-central Mexico have been referred to as Peromyscus boylii; however, a distinct but polymorphic karyotype (fundamental number 54–56) precludes an assignment of these specimens to currently recognized taxa. Phylogenetic analyses (parsimony, likelihood, and Bayesian) of DNA sequences indicated that samples from Durango, Mexico, formed a monophyletic clade that either was sister to P. levipes (parsimony) or to a clade containing P. beatae and P. levipes (likelihood and Bayesian). To refer these samples to P. boylii results in paraphyly and a significantly worse topology. Together, these results indicate that the samples from the Sierra Madre Occidental region of Durango and southeastern Sinaloa represent an undescribed species of Peromyscus. This taxon is described herein as a new species.