Atlantosaurus (meaning "Atlas lizard") is a dubious genus of sauropod dinosaur. The type specimen, found by Arthur Lakes in the Morrison Formation of Colorado, USA, was described by Othniel Charles Marsh, a Professor of Paleontology at Yale University (then called Yale College), in 1877 as "Titanosaurus" montanus. Marsh soon learned that the name Titanosaurus had already been used earlier that year to describe a different sauropod, so he re-named it Atlantosaurus montanus. The skeletal remains discovered were initially distinguished by their immense size and by the pleurocoels (air-filled pockets) in the vertebrae. However, since the time of its discovery, these features have been found to be widespread among sauropods, making it nearly impossible to distinguish the two known vertebrae of Atlantosaurus from those of its relatives. Since it is unclear whether or not Atlantosaurus montanus actually represents a distinct species, it is considered a nomen dubium ("dubious name").
Atlantosaurus was one of the dinosaurs to be described from the United States, during the infamous 19th century Bone Wars, during which scientific methodology suffered in favor of pursuit of academic acclaim.