Ultrasaurus (ultra-sawr-us) lived 100 to 110 million years ago, during the Aptian and Albian ages of the early Cretaceous. It is known from part of an upper forearm (humerus), and some back bones (vertebrae).

It was a long-necked, long-tailed plant-eater with a small head. Fossils have been found in South Korea. Ultrasaurus was named by Kim in 1983; it is a dubious name. The type species is U. tabriensis.

Haang Mook Kim published a paper in 1983, describing a new dinosaur which he named Ultrasaurus tabriensis, because he believed it was an equally giant relative of Jensen's dinosaur. However, Kim's assessment was incorrect. His dinosaur was much smaller than he believed, because he mistook a leg bone (femur) for an arm bone (humerus). However, since Kim was the first to publish the name Ultrasaurus, the name officially applies to the South Korean dinosaur.

Jensen published a paper describing his discovery in 1985, but since the name Ultrasaurus was already in use (preoccupied), his discovery was renamed in 1991 to Ultrasauros. However, Jensen also made a mistake. His discovery was a chimera; the fossils belonged to two different dinosaurs, both of which already had names. So his new name, Ultrasauros, is now just an alternate name (junior synonym) for the dinosaur officially known as Supersaurus.

Kim's Ultrasaurus is currently nomen dubium, which means not enough is known about the specimen to formally assign it to a specific family of sauropods. It may even be a member of a known genus or species, which would make the name Ultrasaurus a junior synonym as well.