Bones of Alamosaurus sanjuanensis were collected from the Upper Cretaceous Javelina and Black Peaks formations in Big Bend National Park, Texas, for use in the first extensive histological study of a titanosaurian sauropod dinosaur. Samples include cervical, dorsal, and caudal vertebrae; ribs; a scapula; humeri; an ulna; a metacarpal; an ischium; femora; a tibia; fibulae; and a metatarsal, representing a minimum of twenty two individuals between 35% and 100% maximum length. In elements of less than 60% maximum length, cortical tissue is primarily reticular or plexiform fibro-lamellar, often with modulations. Resorption in some specimens appears to have proceeded as outwardly expanding bands. In larger specimens (70–79% maximum length), primary tissue is only visible interstitially as either fibro-lamellar or parallel-fibered with low vascularity and annuli. Secondary osteons are in many places linearly arranged. Haversian systems are so pervasive in many of the largest specimens (80–100% maximum length) that virtually no primary tissue remains. However, one adult sized femur possesses laminar fibro-lamellar tissue in its outer cortex, suggesting that the adult size for Alamosaurus may have been underestimated. Camellae typical of titanosaur dorsal vertebrae are present in Alamosaurus but absent in caudal vertebrae. Although pneumatic dorsal ribs characterize Titanosauriformes, sections of ribs analyzed in this study show the cavities in Alamosaurus are much more extensive than most previously described. Overall, Alamosaurus bone histology combines characteristics found in less derived sauropods, such as an early onset of secondary reconstruction, with novel characters such as extensively pneumatic ribs and longer retention of juvenile tissue.