Melanism is a phenotypic trait often driven by selective pressures such as thermoregulation or crypsis. This study reports on a population of bank voles (Clethrionomys glareolus) in the Lepini Mountains (Central Italy) characterized by a remarkably high prevalence of melanistic individuals. While this phenotype was ubiquitous within the study area—exceeding the frequency of the wild-type reddish coloration—it was absent in surrounding regions. Genetic analyses revealed no significant differentiation from neighbouring populations, suggesting that the observed melanism represents a local adaptation rather than a lineage divergence.