Recent investigations in Guinea and Ivory Coast showed that Mus (Nannomys) are hosts of new arenaviruses. However, the taxonomy and biogeography of the western pygmy mice are poorly known, as this genus contains many cryptic species. To improve our taxonomic knowledge on this subgenus and its distribution range in West Africa, we performed a molecular, cytogenetical and morphological study of all Mus (Nannomys) species in this part of Africa. This approach allowed us to detect five species in Guinea (Mus mattheyi, M. minutoides, M. setulosus, M. baoulei, M. musculoides) and four (M. minutoides, M. setulosus, M. baoulei, M. musculoides) in Ivory Coast. Our molecular results confirm the validity of M. baoulei and the extended distribution range of M. mattheyi. We also observed a high degree of genetic differentiation between and within these species, especially within the M. setulosus clade which may be a species complex. The canonical analyses on the craniometrical and external morphological features of the sequenced specimens are insufficient to perfectly separate the studied species. Nevertheless, our results confirm that M. setulosus and M. baoulei are the largest Mus species that occur in Guinea and Ivory Coast. M. baoulei can be distinguished by its short tail and M. setulosus by an enlarged and bilobated lower molar (M3). Among the smaller species, M. musculoides is difficult to identify using morphological data alone, but differences in tail length proportions, size of tympanic bullae, the length and shape of the upper dental row allow identification of this species with some confidence. Our results allow us to provide some conclusions about the distribution ranges of the studied Mus (Nannomys) spp. in both countries and to discuss their presence in relation to vegetation, habitat and human settlements.