The Malagasy mountain mouse or Koopman's montane voalavo (Monticolomys koopmani) is a rodent within the subfamily Nesomyinae of the family Nesomyidae.

Active during the night, it occurs in both montane forest and human-disturbed grasslands and feeds on fruits and seeds.

Although habitat destruction may pose a threat, it is classified as "Least Concern" on the IUCN Red List.

A specimen was captured in 1929 during the Mission Zoologique Franco-Anglo-Américaine to Madagascar, but the rodents obtained by the expedition were never studied in detail.

It was not until the 1970s that Karl Koopman and Guy Musser recognized that the animal—whose skin had landed at the American Museum of Natural History in New York, while the skull was at the Muséum national d'histoire naturelle in Paris—represented an otherwise unknown species.

In 1993, Steven Goodman rediscovered the species on Madagascar and in 1996 he and Michael Carleton finally published a formal description.

[4] Common names in use for the animal also include "Koopman's montane voalavo"[5] and "Malagasy mountain mouse".

[6] Monticolomys, however, does not follow this pattern, in that it is similar and closely related to the gerbil-like genus Macrotarsomys of western Madagascar.

[10] The Malagasy mountain mouse is a small, mouse-like rodent, and quite different in appearance from most other nesomyines.

The cover hairs (which comprise most of the fur) are tricolored: for the basal two-thirds of their length, they are plumbeous gray; the middle is ochraceous; and the tip is dark brown to black.

The zygomatic plate—a bony plate at the side of the skull—is narrow and extends back to the front margin of the first upper molar (M1).

[17] The upper incisors have orange enamel and are opisthodont, with the cutting edge of the tooth inclined backwards.

[21] At Ankaratra, where the species was recorded in 1929, it occurred in such grassland, where the nesomyine Brachyuromys betsileoensis was also found.

However, fires pose a threat in montane forest and, at lower elevations, its habitat is being converted into agricultural land.