Including at least 15 extant species, the beaked whale genus Mesoplodon (Cetacea, Odontoceti, Ziphiidae) is by far the most species-rich cetacean genus. Such a high diversity reflects one or several pulses of radiation, most likely involving a sexual selection mechanism affecting shape, size, and position of mandibular tusks. However, the tempo of this diversification is currently poorly constrained due to the scarce fossil record. Based on the reassessment of five partial skulls discovered in Antwerp (north of Belgium), a new fossil species of the genus Mesoplodon, M. posti, sp. nov., is described. Only the second fossil species of the genus currently recognized, it is characterized (among other features) by: transverse sections of the rostrum higher than wide (a feature proposed to correspond to the presence of lower tusks more posterior than the apex of the mandibles); the presence of a basirostral groove at the rostrum base; pentagonal joined nasals; the posterior projection of the premaxilla in the vertex shorter than the nasal backward; and a strong transverse compression of the frontals behind the nasals. Our phylogenetic analysis results in M. posti being nested among extant species of Mesoplodon. Palynological study of the organic-walled phytoplankton from sediment retained in cranial cavities of several specimens indicates an age between 4.86 and 3.9 Ma (Zanclean, early Pliocene). Constituting the first temporally well-constrained fossil species of Mesoplodon, M. posti provides an upper calibration point for the beginning of the radiation(s), having produced a surprisingly high number of extant species in this genus.