Abstract The Late Campanian-aged Kaiparowits Formation (72.5–76.6 Ma) is a thick, fossiliferous fluvio-overbank–lacustrine complex deposited in south-central Utah. Host to one of the richest, most diverse Campanian terrestrial fossil records in North America, the physical environment of the Kaiparowits ecosystem, particularly the dynamics of the hydrological system, remains obscure. In 2014, an extensive bonebed was found in a conglomeratic sandstone unit of fluvial origin, known as the Rainbows and Unicorns Quarry. Isotopic compositions of serially sampled tooth enamel phosphate (δ 18 O p ) from tyrannosaur teeth were analysed to see if they held data that could clarify the palaeohydrology of the Kaiparowits Formation. To assess a greater palaeoecological context for the tyrannosaurs, the isotopic composition of their teeth was compared to other faunal elements, including the giant alligatoroid Deinosuchus , and turtles (Baenid, Gilmoremys , Neurankylus and Trionychid) from the same quarry. The δ 18 O w calculated from tyrannosaur was low, suggesting that isotopically-light, high-altitude runoff strongly influenced the local hydrology of the Kaiparowtis Formation. The seasonal change in δ 18 O of tyrannosaur drinking water ranged from −21.0 to −14.4‰ V-SMOW, while δ 18 O w calculated from turtles and crocodiles ranged between an average of −9.3 and −10.9‰, respectively. The seasonal precipitation pattern and temperature of the Kaiparowits Formation was then compared to analogue settings, such as the monsoonal climate of Hanoi, Vietnam and Cuiaba, Brazil. While similarities exist between these sites, the unique configuration of the Sevier Mountains adjacent to the nearby Western Interior Seaway provides the unique setting for a complex palaeohydrological system that results in a wide range of δ 18 O w as a result of cold seasonal precipitation at high elevation that runs off and mixes with local precipitation (−6.0 to −4.68‰) sourced from the Western Interior Seaway. This study demonstrates the importance of serial- and micro-sampling of multi-taxa comparisons for isotopic investigations of palaeohydrological systems.