Tenontosaurus (ten-ONT-oh-SAWR-us) was a very large (24 feet long and about 500 kg or 1 ton in weight) hypilophodontid that lived in the Aptian-Albian ages in the Lower Cretaceous period about 116-113 million years ago. They lived in prehistoric swamps.

Most hypsilophodontids were a lot smaller than Tenontosaurus and ran on two legs - they also had much smaller arms than Tenontosaurus. Tenontosaurus' front legs were a little smaller than its rear legs. Although it probably ran on two legs, Tenontosaurus was large and bulky and probably spent most of its time on four legs, grazing.

Tenontosaurus was named in 1970 by John H. Ostrom and G. E. Meyer.