Brachiosaurs were built like huge giraffes; they had immensely long necks and relatively short tails. Their morphology is unusual among dinosaurs in that the forelimbs were longer than the hind limbs. These adaptations apparently enabled them to lift their heads to about 12 metres (39 feet) above the ground in order to browse the branches of tall trees. Brachiosaurs attained a maximum length approaching 25 metres and a weight of nearly 80 metric tons. Their nasal bones were expanded into a broad arch that presumably allowed them to maintain some distance between the vegetation and the nasal openings so that they could breathe easily while feeding. The mouth contained a few dozen pencil-like teeth with beveled edges. Like most other dinosaurs, brachiosaurs did not chew their food but used their jaws to collect food, which the tongue presumably forced into the throat. Considering their massive size, their small heads, and the relatively poor quality of their forage, scientists have inferred that brachiosaurs must have spent nearly all their waking hours feeding.

The huge size of brachiosaurs led some researchers to suggest that they spent most of their time submerged in water, which would have served to buoy up their great weight. The location of the nasal openings—on top of the head and above the eyes—lent additional support to this idea. However, water pressure at the depths needed to cover these dinosaurs would have crushed their lungs and thus made breathing difficult or impossible. Other features of their skeleton show that brachiosaurs were well adapted to a life spent on land browsing the high treetops. Their skeletons were strong but not massive, so their weight could be supported without any help from water. Their great neck bones, for example, are so deeply excavated that they function as a lightweight framework of struts and plates.