A new styracosternan ornithopod genus and species is described based on the right dentary of a single specimen from the Mirambell Formation (Early Cretaceous, early Barremian) at the locality of Portell, (Castellon, Spain). Portellsaurus sosbaynati gen. et sp. nov. is diagnosed by two autapomorphic features as well as a unique combination of characters. The autapomorphies include: the absence of a bulge along the ventral margin directly ventral to the base of the coronoid process and the presence of a deep oval cavity on the medial surface of the mandibular adductor fossa below the eleventh-twelfth tooth position. Phylogenetic analyses reveal that the new Iberian form is more closely related to the African taxon Ouranosaurus nigeriensis than to its synchronic Iberian taxa Magnamanus soriaensis and Iguanodon galvensis. In addition, Portellsaurus sosbaynati is less related to other Iberian taxa such as Iguanodon bernissartensis and Proa valdearinnoensis than to the other Early Cretaceous Iberian styracosternans Mantellisaurus atherfieldensis and Morelladon beltrani. A new phylogenetic hypothesis is proposed that resolves Iguanodon (I. bernissartensis, I. galvensis) with the Valanginian Barilium dawsoni into a monophyletic clade (Iguanodontoidea). The recognition of Portellsaurus sosbaynati gen. et sp. nov. as the first styracosternan dinosaur species identified from the Margas de Mirambell Formation (early Barremian-early late Barremian) in the Morella sub-basin (Maestrat Basin, eastern Spain) indicates that the Iberian Peninsula was home to a highly diverse assemblage of medium-to-large bodied styracosternan hadrosauriforms during the Early Cretaceous.