Ichthyosauria, Plesiosauria, and Metriorhynchidae were apex predators in Mesozoic oceanic trophic networks. Previous stable oxygen isotope studies suggested that several taxa belonging to these groups were endothermic and that some of them were homeothermic organisms. However, these conclusions remain contentious owing to the associated uncertainties regarding the δ18O value and oxygen isotope fractionation relative to environmental seawater. Here, we present new bioapatite phosphate δ18O values (δ18Op) of Ichthyosauria, Plesiosauria, and Metriorhynchidae (Middle Jurassic to Early Cretaceous) recovered from mid- to high paleolatitudes to better constrain their thermophysiology and investigate the presence of regional heterothermies. The intraskeletal δ18Op variability failed to reveal distinct heterothermic patterns within any of the specimens, indicating either intrabody temperature homogeneity or an overriding diagenetic overprint of the original biological δ18Op bone record. Body temperature estimates have been reassessed from new and published δ18Op values of well-preserved isolated teeth, recently revised Mesozoic latitudinal δ18O oceanic gradients, and 18O-enrichment factors of fully aquatic air-breathing vertebrates. Our results confirm that Ichthyosauria were homeothermic endotherms (31°C to 41°C), while Plesiosauria were likely poikilothermic endotherms (27°C to 34°C). The new body temperature estimates of the Metriorhynchidae (25°C to 32°C) closely follow ambient temperatures and point to poikilothermic strategy with no or little endothermic ability. These results improve our understanding of Mesozoic marine reptile thermoregulation and indicate that due to their limited body temperature variations, the δ18Op values from Ichthyosauria fossil remains could be used as valuable archives of Mesozoic oceans δ18Osw values that may help improve paleoenvironmental and paleoclimatic reconstructions.