| Named By: | P. M. Galton in 2007 |
| Time Period: | Late Jurassic, 155-148 Ma |
| Location: | USA - Morrison Formation rocks in Utah and Wyoming |
| Size: | About 2 meters long |
| Diet: | Herbivore |
| Fossil(s): | Several individuals |
| Classification: | | Chordata | Reptilia | Dinosauria | Ornithischia | Genasauria | Neornithischia | |
| Also known as: | | Laosaurus consors | |
Othnielosaurus is a genus of ornithischian dinosaur that lived about 155 to 148 million years ago, during the Late Jurassic-age Morrison Formation of the western United States. It is named in honor of famed paleontologist Othniel Charles Marsh, and was formerly assigned to the genus Laosaurus. This genus was coined to hold fossils formerly included in Othnielia, which is based on remains that may be too sparse to hold a name; as such, it is part of decades of research to untangle the taxonomy left behind by Marsh and his rival Edward Drinker Cope from the Bone Wars. Othnielosaurus has usually been classified as a hypsilophodont, a type of generalized small bipedal herbivore or omnivore, although recent research has called this and the existence of a distinct group of hypsilophodonts into question.