Thyreosaurus is an extinct genus of stegosaurian dinosaur from the Middle Jurassic El Mers III Formation of Morocco.

The generic name, Thyreosaurus, combines thyreos (θυρεός), the name of an oblong shield used in Ancient Greece—referencing the specimen's unusual osteoderms—with the Greek σαῦρος (sauros), meaning "lizard".

[1] The holotype of Thyreosaurus belonged to an adult individual with an estimated body length of 6 metres (20 ft), but it was not fully grown.

It possessed unique dermal armor that has been compared to that of nodosaurid ankylosaurs,[4] consisting of asymmetrically sided oblong plates that were likely arranged recumbent on the back of the animal, instead of being held erect as in other stegosaurs.

[1] In their phylogenetic analysis, Zafaty et al. (2024) recovered Thyreosaurus as dacentrurine member of the Stegosauridae, as the sister taxon to Dacentrurus.