In recent years, the discovery of isolated teeth from the Late Cretaceous (Campanian-Maastrichtian) of the Basque-Cantabrian Region, in the north of the Iberian Peninsula, has permitted to recognise at least four different mosasaurid taxa in several sites of Alava and Condado de Treviño: Mosasaurus lemonnieri, Prognathodon solvayi, Platecarpus cf. ictericus and Tylosaurus sp. A new specimen, which consists of a fragment of skull with articulated portions of the maxilla and prefrontal, from the Campanian of Navarre is described here. This material is assigned to Prognathodon cf. sectorius on the basis of dental features. It constitutes the only cranial remain and the most significant mosasaurid fragment known to date in the Iberian Peninula. P. sectorius was previously recorded from the Maastrichtian of New-Jersey and The Netherlands.