Alamosaurus was a gigantic quadrupedal herbivore with a long neck and tail and relatively long limbs. Its body was at least partly covered in bony armor. Though most of the complete remains come from juvenile or small adult specimens, one fragmentary specimen suggests that adult Alamosaurus could have grown to enormous sizes comparable to the largest known dinosaurs like Argentinosaurs, which has been estimated to weigh 73 tonnes (72 long tons; 80 short tons).
Though no skull has ever been found, rod-shaped teeth have been found with Alamosaurus skeletons and probably belonged to this dinosaur. The vertebrae from the middle part of its tail had elongated centra. Alamosaurus had vertebral lateral fossae that resembled shallow depressions. Fossae that similarly resemble shallow depressions are known from Saltasaurus, Malawisaurus, Aeolosaurus, and Gondwanatitan. Venenosaurus also had depression-like fossae, but its "depressions" penetrated deeper into the vertebrae, were divided into two chambers, and extend farther into the vertebral columns. Alamosaurus had more robust radii than Venenosaurus.