A new species of spiny mouse, Mus (Pyromys) dumbara sp. nov., is described from the Dumbara (Knuckles) Mountain Range in Sri Lanka, based on an integrated assessment of external morphology, cranial characteristics, mitochondrial and nuclear DNA sequence data. This species is assigned to the subgenus Pyromys on the basis of two defining cranial characteristics: the presence of a supraorbital ridge and incisive foramina that extend to the mid-length of first upper molar. Mus dumbara sp. nov. is characterised by a tail distinctly longer than its combined head and body length and a moderately prominent supraorbital ridge which is clearer at the junction between parietal and frontal upon the orbit. There are several other external and cranial characteristics which can be used to distinguish M. dumbara sp. nov. from each pyromys species. Genetic analysis further confirms the distinctiveness of M. dumbara sp. nov. Mitochondrial cytochrome-b sequences reveal deep divergence from other Sri Lankan spiny mice (M. mayori and M. fernandoni), with uncorrected pairwise genetic distances exceeding 11.7%. This level of genetic separation, combined with its distinctive morphology and geographically restricted distribution in the Dumbara valley, provides strong evidence for its status as a new species endemic to Sri Lanka.