| Named By: | Peter M. galton in 1977 |
| Time Period: | Late Jurassic, 150 Ma |
| Location: | USA |
| Size: | Uncertain but probably somewhere in the region of 1.5 to 2 meters long |
| Diet: | Herbivore |
| Fossil(s): | Femur |
| Classification: | | Chordata | Reptilia | Dinosauria | Ornithischia | Neornithischia | |
| Also known as: | | Nanosaurus rex | |
Othnielia is a genus of ornithischian dinosaur, named after its original describer, Professor Othniel Charles Marsh, an American paleontologist of the 19th century. The taxon, Othnielia rex, was named by Peter Galton in 1977 from a species Marsh (1877) called Nanosaurus rex.