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The upper Maastrichtian Breien Member situated within the lower portion of the Hell Creek Formation in south-central North Dakota records one of the last transgressions of the Western Interior Seaway (WIS) during the terminal Cretaceous. A fragmentary articular-prearticular complex and isolated vertebra belonging to a mosasauroid were recovered in 2016 from sandstones and mudstones deposited in a nearshore marine paleoenvironment within the southern arm of the bisected WIS. The medially-rotated retroarticular process on the articular-prearticular complex, the shape of the glenoid fossa, along with the morphology of the isolated vertebra, facilitate a conservative referral to a large-bodied mosasaurine such as Mosasaurus or Prognathodon. The rocks of the Breien Member provide paleontologists a unique glimpse of intracontinental marine ecosystems immediately prior to the end of the Cretaceous Period. This discovery provides additional evidence that the latest Maastrichtian marine fauna is a continuation of the fauna preserved in the underlying Fox Hills Formation and that the marine faunal turnover that gave rise to the subsequent Cannonball Sea fauna recorded in Paleocene rocks in North Dakota occurred at the Cretaceous-Paleogene boundary.