Three juvenile specimens of Prosaurolophus maximus, represented by articulated to disarticulated skeletons, are the smallest known individuals for the taxon. Cranial anatomy of the juvenile specimens indicates that diagnostic characters of P. maximus are ontogenetically variable. In the smallest individual, the crest and deeply excavated fossa at the caudal margin of the circumnarial depression are poorly developed or absent. The crest approaches adult-like morphology in large juveniles, whereas crest robusticity and the deep excavation of the circumnarial depression occur only in subadult and adult individuals. The shape of the caudal margin of the circumnarial depression is consistent between juvenile and adult individuals, potentially making this feature a reliable character for taxonomic identification at younger ontogenetic stages. The crest of P. maximus grows isometrically during ontogeny, unlike the positive allometric growth of lambeosaurine hadrosaur crests, suggesting that this taxon may have had soft tissue structures associated with the narial-crest region, rather than the bony crest itself, selected for sexual display. Recovered from sediments of the Bearpaw Formation deposited during the Baculites compressus ammonite zone and magnetochrons 33n.3n to 33n.2n, the juvenile specimens are stratigraphically younger than P. maximus specimens from the Dinosaur Park Formation (Alberta) and contemporaneous with most specimens from the Two Medicine Formation (Montana), extending the temporal range of the taxon to 75.7–74 Ma. The occurrence of P. maximus in well-drained terrestrial deposits of the Dinosaur Park and Two Medicine formations and marine sediments of the Bearpaw Formation indicates that this taxon inhabited various paleoenvironments in western North America.