The Milk River Transboundary Aquifer (Canada–USA) has been so intensively used over the 20th century that concerns have risen about the durability of this resource since the mid-1950s. This aquifer actually corresponds to the middle Virgelle Member of the Upper Cretaceous Milk River Formation (called Eagle Formation in Montana). To assess the conditions needed for a sustainable use of the aquifer, a comprehensive and unified portrait of the aquifer is needed across its international boundary. The stratigraphic framework and geometry of geological units on both sides of the international border were thus unified in a 50 000 km2 three-dimensional (3D) geological model. The Virgelle Member is 0–60 m thick and it subcrops near the border and along both sides of the Sweetgrass Arch. It dips away from the subcrop areas in a semi-radial pattern. The Medicine Hat gas field hosted by the Alderson Member (Alberta), which is separated from the other members by a regional unconformity, and the Tiger Ridge gas field near the Bears Paw Mountains (Montana) limit the extent of the aquifer. The unified 3D geological model forms the necessary basis for conceptual and numerical hydrogeological models of the Milk River Aquifer.