| Named By: | Othniel Charles Marsh in 1877 |
| Time Period: | Unavailable |
| Location: | USA, Colorado - Morrison Formation. Maybe also Wyoming |
| Size: | Uncertain due to lack of good fossil remains, but possibly somewhere between 1 and 2 meters long |
| Diet: | Herbivore |
| Fossil(s): | Based upon partial dentary and fragmentary remains of the skull and post cranial skeleton |
| Classification: | | Chordata | Reptilia | Dinosauria | Ornithischia | Nanosauridae | |
Nanosaurus ("small or dwarf lizard") is the name given to a genus of dinosaur from the Late Jurassic. Described by Othniel Charles Marsh in 1877, it is a poorly known ornithischian of uncertain affinities. Its fossils are known from the Morrison Formation of Colorado and possibly Wyoming. It has often been illustrated in the popular literature (as a "tiny dinosaur"), leaving the impression that more is known about it than actually is. Most representations are actually of what is now called Othnielosaurus or Othnielia.