Karydomys is a rare and little diversified democricetodontine, of which only six species are currently recognized. This group of rodents is first recorded in the early Miocene (MN3) in China and spread quickly thereafter to Kazakhstan and Greece (MN4). Karydomys reached south-western and central Europe by early middle Miocene times (MN5), from where it became extinct shortly thereafter (MN6). A new species of Karydomys is here described from the Miocene Keramia Formation of Chios Island (north-eastern Aegean Sea). Karydomys strati sp. nov. is characterized by the presence of a labial spur on the anterolophule, the lack of anterior protolophule on the M1 and by the presence of a double metalophule on the M2. A cladistics analysis involving all the species of Karydomys and some closely related species of Democricetodon and Cricetodon show that Karydomys split early into two different lineages, an ‘eastern stock’, which includes the central Asian (K. debruijni and K. dzerzhinskii) and Greek (K. symeonidisi, K. boskosi and K. strati) species, and a ‘western lineage’ comprising the western and central European species (K. wigharti and K. zapfei). The evolutionary stage of Karydomys strati suggests that the lowermost part of the Keramia Formation, usually attributed to the middle Miocene, is older than previously thought.