The Supersaurus was a typical diplodocid, with a small head, solid legs, long neck and tail. It could have measured 35m long, which would make the Supersaurus one of the longest animals that ever have walked the Earth, only surpassed by the Seismosaurus. Originally, it was thought that Supersaurus was related to the long-necked diplodocid Barosaurus, and therefore a member of the subfamily Diplodocinae, though most later studies found Supersaurus to be a close relative of the familiar Apatosaurus in the group Apatosaurinae. Like the Diplodocus, it had spines in its back.
The original fossil remains of Supersaurus, discovered in the Dry Mesa Quarry, yielded only a few bones: the shoulder girdle, an ischium and a few neck vertebrae. This shoulder girdle stood some 2.4 meters (8 ft) tall, if placed on end. A new and much more complete specimen of Supersaurus, nicknamed 'Jimbo', was found in Converse County, Wyoming in 1996. It is currently being prepared and was described in 2007. Its bones are being held at the Wyoming Dinosaur Center. By comparing the two specimens, it could be established that a series of tail vertebrae referred to Supersaurus by Jensen may have belonged to some other form. Paleontologist James A. Jensen, who described the original Supersaurus specimen, simultaneously reported the discovery of another gigantic sauropod, which would later be named Ultrasaurus macintoshi (later renamed Ultrasauros macintoshi). The type specimen (the specimen used to define a new species) of Ultrasauros, being a backbone (dorsal vertebra), was later found to have come from Supersaurus. In fact, it probably belonged to the original Supersaurus specimen, which was discovered in the same quarry in 1972. Therefore, Ultrasauros became a junior objective synonym of Supersaurus, and it is not a junior synonym for Brachiosaurus. Since Supersaurus was named slightly earlier, the name Ultrasauros has been discarded in favor of Supersaurus. Other bones that were found at the same location and originally thought to belong to Ultrasauros, like a shoulder girdle, actually belonged to Brachiosaurus, possibly a large specimen of Brachiosaurus altithorax. The Brachiosaurus bones indicate a large, but not record-breaking individual, a little larger than the Giraffatitan brancai mount in the Humboldt Museum of Berlin. Larger specimens of Brachiosaurus are known from the Tendaguru beds of Tanzania, in east Africa. Originally, these Supersaurus and Brachiosaurus bones were believed to represent a single dinosaur that was estimated to reach about 24.3 to 30 meters (80 to 100 ft) long, 7.6 meters (25 ft) high at the shoulder, 15.2 meters (50 ft) in total height, and weighing maybe 70 metric tons (75 short tons).
The Supersaurus lived like the other sauropods. Its long neck was probably used to eat leaves of trees of the forest, without entering the rest of the body in it. Like the rest of sauropods, it had a small head, with a small brain, and a long tail which it probably used to defend itself against predators like Allosaurus, although these theropods maybe didn't try to hunt them because of the Supersaurus incredible size. The Supersaurus also could rear into a bipedal pose to seem bigger, increasing its size. But they had to protect their sons, because they were much smaller than its fathers, and they probably were an easy prey for the predators.