Monticolomys is a genus of rodents within the subfamily Nesomyinae of the family Nesomyidae, and is closely related to Macrotarsomys. The only species, Monticolomys koopmani, also known as the Malagasy mountain mouse or Koopman's montane voalavo, is found in the highlands of eastern Madagascar. A small mouse-like rodent, M. koopmani is dark brown on the upperparts and dark gray below. It has small, rounded, densely haired ears and broad feet with well-developed pads. The long tail lacks a tuft at the tip. The skull is delicate and lacks crests and ridges on its roof.