Ultrasaurus (meaning "ultra lizard"[2]) is a genus of sauropod dinosaur discovered by Haang Mook Kim in South Korea.

However, the name was first used unofficially (as a nomen nudum) in 1979 by Jim Jensen to describe a set of giant dinosaur bones he discovered in the United States.

A collection of bones discovered by Jim Jensen, of Brigham Young University, at the Dry Mesa Quarry, Colorado were originally believed to belong to the largest dinosaur ever.

In 1983, Kim Hang-mook published a paper describing a different specimen representing a new dinosaur species, which he named Ultrasaurus tabriensis, because he believed it was bigger than Supersaurus.

It is known from the holotype DGBU-1973, which consists of part of an upper forearm (humerus), and DGBU-1978-A, a single tailbone (vertebrae) from the Gugyedong Formation, belonging to the Hayang Group.