| Named By: | George Olshevsky in 1991 |
| Time Period: | Middle Jurassic |
| Location: | China, Sichuan Province - Shaximiao Formation |
| Size: | Uncertain due to lack of post cranial remains |
| Diet: | Herbivore |
| Fossil(s): | Lower jaw |
| Classification: | | Chordata | Reptilia | Dinosauria | Saurischia | Sauropoda | Euhelopodidae | |
| Also known as: | | Protognathus oxyodon | |
Protognathosaurus (meaning "early jaw lizard") is a genus of herbivorous dinosaur from the Middle Jurassic. It was a sauropod found at Dashanpu in Sichuan in what is present-day China.
In 1988 Zhang Yihun named and described the type species Protognathus oxyodon. The generic name was derived from Greek protos, protos, "first", and gnathos, gnathos, "jaw". The specific name is derived from Greek oxus, oxys, "sharp", and odon, odon, "tooth". However, the generic name was preoccupied, already in use, by a genus of beetle, Protognathus Basilewsky 1950. The species was therefore renamed into the combinatio nova Prognathosaurus oxyodon by George Olshevsky in 1991.
Protognathosaurus is known only from a fragmentary lower jaw, holotype CV 00732 (= ChM V732) found in a layer of the Shaximiao Formation dating from the Bathonian-Callovian. The fragment consists of the front left dentary and shows eighteen tooth positions, some still containing sharp replacement teeth, after which the genus was named. Most researchers consider Protognathosaurus to be a nomen dubium.