Smithsonian paleontologist Gilmore originally described the holotype, USNM 10486, a left scapula (shoulder bone), and the paratype USNM 10487, a right ischium (pelvic bone) in 1922, naming the type species Alamosaurus sanjuanensis.
In 1946, Gilmore posthumously described a more complete specimen, USNM 15660 found on 15 June 1937 on the North Horn Mountain of Utah by George B. Pearce.