Brachiosaurus is a genus of sauropod dinosaur that lived in North America during the Late Jurassic, about 154-153 million years ago.

Based on a complete composite skeleton, Brachiosaurus attained 25 meters (82 ft) in length and was probably able to raise its head about 13 meters (43 ft) above ground level. However, fragmentary material from larger specimens suggests that it could grow 15% longer than this, giving it a maximum possible length of 29 meters (94 ft).

Its diet likely consisted of ginkgos, conifers, tree ferns, and large cycads, with intake estimated at up to 400 kilograms (880 lb) of plant matter daily. If it fed sixteen hours per day, biting off between a tenth and two-thirds of a kilogram, taking between one and six bites per minute, its daily food intake would have equaled roughly 1.5% of its body mass, comparable to the requirement of a modern elephant.

The name comes from the Greek words brachio meaning "arm" and saurus meaning "lizard"--a reference to its proportionately long arms.

It was first described by American paleontologist Elmer S. Riggs in 1903 from fossils found in the Colorado River valley in western Colorado.

It was believed throughout the 19th and early 20th centuries that sauropods like Brachiosaurus were too massive to support their own weight on dry land, and instead lived partly submerged in water. Riggs was the first to defend in length that most sauropods were fully terrestrial animals, pointing out that their hollow vertebrae have no analogue in living aquatic or semiaquatic animals, and their long limbs and compact feet indicate specialization for terrestrial locomotion. Though Riggs' ideas were originally dismissed, the notion of sauropods as terrestrial animals is now universally accepted among paleontologists.

The overall build of Brachiosaurus resembles a giraffe more than any other living animal. In fact, a closely related brachiosaurid dinosaur from the Tendaguru Formation of Tanzania was formally named Giraffatitan.

It has been proposed that sauropods, including Brachiosaurus, may have had proboscises (trunks) based on the position of the bony narial orifice, to increase their upward reach. Fabien Knoll and colleagues disputed this for Diplodocus and Camarasaurus in 2006, finding that the opening for the facial nerve in the braincase was small. The facial nerve was thus not enlarged as in elephants, where it is involved in operating the sophisticated musculature of the proboscis. However, Knoll and colleagues also noted that the facial nerve for Giraffatitan was larger, and could therefore not discard the possibility of a proboscis in this genus.

Compared to its hind limbs, the relatively long length of its front limbs contributed to this dinosaur's distinctly giraffe-like posture. This was probably a dietary adaptation, as the longer front limbs allowed Brachiosaurus to reach the high branches of trees without straining its neck.

It has been estimated that the huge sauropods, like Apatosaurus, Brachiosaurus, Diplodocus, and Supersaurus lived to be roughly 100 years old.