Iguanodontia is a diverse clade of herbivorous ornithischian dinosaurs that were speciose and abundant during the Jurassic and Cretaceous. Although the monophyly of Iguanodontia is well supported, their internal relationships have sparked heated debate due to several phylogenetic paradigm shifts. Late Jurassic basally branching iguanodontians in particular are not well understood in terms of their systematic affinities and evolutionary relevance. Their fossil record in Europe is meager compared with North America, with only a few species currently recognized. Two taxa are currently known from the Upper Jurassic of England, the basally branching styracosternan Cumnoria prestwichii and the putative dryosaurid Callovosaurus leedsi. In the Upper Jurassic of Portugal, the styracosternan Draconyx loureiroi and the dryosaurid Eousdryosaurus nanohallucis are presently the only described basally branching iguanodontians. Here we report a new species of early diverging iguanodontian from the Upper Jurassic Lourinhã Formation of western-central Portugal. The new species is clearly distinguished from all other coeval taxa by an exclusive combination of characters that include a tibia with a cnemial crest that is directed craniolaterally and a fibular condyle that is angled at 90° with respect to the proximal epiphysis, a fibula with symmetrical proximal margins, and a reduced metatarsal I. The phylogenetic relationships of the Lourinhã iguanodontian were explored using maximum parsimony and Bayesian inference. The two analyses recover the Lourinhã iguanodontian as an indeterminate dryomorphan, with more precise affinities precluded due to the current available material. Body size is estimated between 3 and 4 meters for the holotype specimen, adding to the diversity of small ornithopods already recognized in the paleoichnological record of the Lourinhã Formation. http://zoobank.org/urn:lsid:zoobank.org:pub:5F4D52D0-0F0B-4809-8561-BE58C7C97D45

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