Alamosaurus, meaning "Ojo Alamo lizard," is a genus of opisthocoelicaudiine titanosaurian sauropod dinosaurs. It includes a single known species, Alamosaurus sanjuanensis, which lived during the Maastrichtian age of the Late Cretaceous period in what is now southwestern North America. Fossils of isolated vertebrae and limb bones suggest that it grew to sizes comparable to Argentinosaurus and Puertasaurus, making it the largest dinosaur ever discovered in North America.

Alamosaurus was an awe-inspiring dinosaur, reaching lengths of up to 98 feet (30 meters) and standing approximately 40 feet (12 meters) tall at the shoulder. With its elongated neck and tail, it epitomized the typical sauropod body plan. Its forelimbs were shorter than its hind limbs, and all four limbs supported its massive body weight. The neck, in particular, was remarkably long and allowed Alamosaurus to reach vegetation high above the ground.

Like other Titanosaurs, Alamosaurus had a relatively small head compared to the immense size of its body. Its teeth were continuously replaced throughout its lifetime, enabling it to efficiently chew tough plant material, such as ferns and cycads, which were abundant in its Late Cretaceous environment.

Alamosaurus holds a unique distinction as the sole known sauropod to have lived in North America following the sauropod hiatus, a significant gap of nearly 30 million years during which no definite sauropod fossils were discovered on the continent. The earliest fossils of Alamosaurus date back to approximately 70 million years ago, during the Maastrichtian age. Soon after its appearance, Alamosaurus swiftly established itself as the dominant large herbivore in the region of southern Laramidia.