Molecular, chromosomal and morphometric analyses of wild-caught mice of subgenus Mus from the central plain of Thailand are presented. These specimens are distinct from all species previously described in the literature. This has led to the characterization of Mus fragilicauda sp. n., a new member of the set of closely related species encompassed by the subgenus. While this species may be considered as a sibling and sympatric species of the Asian M. cervicolor, M. fragilicauda sp. n. is phylogenetically closer to the M. musculus complex of species and to the other European species of Mus.