| Named By: | Masateru Shibata, Pratueng Jintasakul & Yoichi Azuma in 2011 |
| Time Period: | Early Cretaceous, Aptian |
| Location: | Thailand, Nakhon Ratchasima Province - Khok Kruat Formation |
| Size: | Unknown |
| Diet: | Herbivore |
| Fossil(s): | Dentary (lower jaw) |
| Classification: | | Chordata | Reptilia | Dinosauria | Ornithischia | Ornithopoda | Igaunodontia | |
Ratchasimasaurus is a genus of non-hadrosaurid iguanodontian ornithopod dinosaur from Early Cretaceous (Aptian stage) Khok Kruat Formation of Nakhon Ratchasima Province in northeastern Thailand. One autapomorphy (unique characteristic) of Ratchasimasaurus is its elongated and flat ramus of the dentary. Ratchasimasaurus shows both primitive and derived characters for Iguanodontia, such as "a caudally inclined coronoid process and alveolar trough with a primitive crown impression, and a derived buccal shelf between the tooth row and the coronoid process."