Three partial skulls of odontocetes from the Miocene of Antwerp and Kessel, northern Belgium (southern North Sea Basin) are described here as a new genus and species of beaked whale (Ziphiidae), Archaeoziphius microglenoideus. This small new whale is one of the most archaic beaked whales, because of its low vertex, thin and laterally directed premaxillary crests, low contact between the supraoccipital and the frontals on the vertex, and its free atlas. It is also characterized by the strong transverse compression of the frontals on the vertex and a reduced glenoid surface on the squamosal; the two last characters might be considered as derived. Nasals wider than the frontals on the vertex might group A. microglenoideus with the extant larger genera Berardius and Tasmacetus; the nodular frontals and the premaxillary crests are more similar to the condition in Berardius. A sediment sample from the cetacean-bearing strata at Kessel is dated with dinoflagellate cysts as middle Miocene, tentatively late Langhian to early Serravallian, i.e., from ca. 15 Ma to ca. 13.2 Ma. Archaeoziphius microglenoideus is therefore the oldest reported beaked whale known by cranial material.