Understanding the causes of the low-biodiversity faunas in the Western Interior Seaway (WIS), overwhelmingly dominated by ammonite and bivalve molluscs, will help us to understand paleoenvironmental conditions in the WIS. In this study, we examined rare earth elements (REEs) in well-preserved molluscs from the Bearpaw Formation to reconstruct WIS redox conditions. Both nektonic and benthic molluscs share similar REE patterns with enrichment in light REEs. There is only a slight Ce depletion in both types of molluscs, indicating no significant fractionation of Ce from the other REEs. A lack of significant Ce anomalies in molluscs points to oxygen-deficient (probably dysoxic) conditions in the middle to bottom part of the water column where the molluscs lived. Given the general lack of significant Ce anomalies also in molluscs from older formations, oxygen deficiency was likely prevalent in the Late Cretaceous in the WIS. A warmer climate in the Cretaceous is probably a driver of such conditions, as predicted by ocean models. Long-term oxygen deficiency and stratification, inferred from the heterogeneity in δ18O and δ13C values of molluscs, may also imply weak circulation, reducing the exchange of surface water and subsurface water, and the transport of oxygen into the WIS. The oxygen deficiency and weak circulation of the seaway is also expressed in the general characteristics of faunas and possibly Bearpaw sedimentary rocks. Oxygen deficiency rather than the brackish water conditions may have been largely responsible, therefore, for the low diversity of the WIS fauna.