Othnielia is a genus of ornithischian dinosaur, named after its original describer, Professor Othniel Charles Marsh, an Americanpaleontologist of the 19th century. The taxon, Othnielia rex, was named by Peter Galton in 1977 from a species Marsh (1877) called Nanosaurus rex.

Othnielia and two partial skeletons were specifically dealt with in Galton's paper, leaving unsettled the assignment of several other specimens that have appeared in the literature. Included among these are a nearly complete specimen in the Aathal Dinosaur Museum nicknamed "Barbara", and a dentary (MWC 5822, again referred to O. rex). That Galton considered Othnielia a nomen dubium means that, according to him, these other specimens could not be referred to it.

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