Ultrasaurus, meaning “Ultra lizard,” is a genus of sauropod dinosaur from the Aptian and Albian stages of the Early Cretaceous Period (110 to 100 mya). It was discovered by Haang Mook Kim in South Korea. It is known from part of an upper forearm and some back bones.

Ultrasaurus was first unofficially used in 1979 by Jim Jensen to describe a set of dinosaur bones he discovered in the United States. Because Kim published the name for his specimen before Jensen could do so officially, Jensen renamed his specimen Ultrasaurus. It was later found to be a chimera, and Jensen’s specimen is now assigned to Supersaurus.

Haang Mook Kim published a paper in 1983 describing a different specimen representing a new dinosaur genus, which he named Ultrasaurus tabriensis, because he believed it was an equally giant relative of Jensen’s specimen. However, Kim’s assessment was incorrect. His dinosaur was much smaller than he believed, because his mistook a partial humerus for an ulna. But since Kim was the first to publish the name Ultrasaurus, the name officially applied to his small South Korean sauropod, and could no longer be used for Jensen’s giant dinosaur.

Kim’s Ultrasaurus is currently classified as nomen dubium, meaning there is not enough known about the specimen to formally assign it to a specific family of sauropod. It may even be a member of an already known genus or species, which would make the name Ultrasaurus a junior synonym as well.