Based on morphological and morphometric comparisons, a new species of Hylomyscus, H. arcimontensis (Muridae: Murinae), is identified from Mount Rungwe and the Eastern Arc Mountains of central and eastern Tanzania. The new species is confined to wet montane forests of these mountains and represents another vertebrate endemic to the Tanganyika-Nyasa Montane Forest biotic region (sensu Moreau 1966). It is most closely related to H. anselli, a form described from mountains in northern Zambia as a subspecies of H. denniae and here elevated to species rank. Morphological evidence supports the view that the nominal species H. denniae is a composite of several species whose interrelationships and differentiation patterns indicate two species complexes: the H. denniae group centered in Central East African mountains (Burundi, Kenya, Rwanda, Uganda, Zaire) and the H. anselli group distributed across more southern mountains (Angola, Malawi, Tanzania, Zambia).