The Cretaceous-Paleogene (K-Pg) mass extinction has been attributed to the impact of a large bolide at the end of the Cretaceous Period, although other potential causes have also been proposed, most notably climate change caused by Deccan Traps (India) flood volcanism. Reconstructing paleoclimate, particularly in terrestrial settings, has been hindered by a lack of reliable proxies. The recent development of carbonate clumped isotope paleothermometry has contributed to temperature reconstructions using geochemical proxies in terrestrial settings. We employ this method, along with new stratigraphic constraints, in the Hell Creek (Cretaceous) and overlying Fort Union (Paleogene) Formations (Montana, USA) to examine changes in temperature leading to and across the K-Pg boundary. We demonstrate that well-preserved ca. 66 Ma aragonitic bivalves serve as suitable paleoclimate archives. Although there are limitations in the stratigraphic availability of fossil bivalves for clumped isotope analysis, we record an apparent 8 °C decrease in summer temperatures over the last 300 k.y. of the Cretaceous that corresponds broadly with patterns observed in other paleotemperature proxies. This observed decrease plausibly could be explained by an absolute temperature decrease or by other environmental stresses on the organisms, but in either case suggests changing living conditions over the interval. Previously documented declines in vertebrate and invertebrate biodiversity occur over the same stratigraphic interval at this study location. These results are consistent with published models of the end-Cretaceous mass extinction in which destabilized ecosystems become more susceptible to an abrupt event like a bolide impact.