We describe a new species of nanhsiungchelyid turtle, Basilemys morrinensis, based on a nearly complete shell from the Horsethief Member (lower Maastrichtian) of the Horseshoe Canyon Formation of Alberta. The species is intermediate in age between the Campanian forms B. variolosa and B. gaffneyi and the upper Maastrichtian forms B. sinuosa and B. praeclara. It is also intermediate in its morphology, possessing a unique suite of both plesiomorphic (e.g., divided extragulars) and derived (e.g., square epiplastral beak, pygal wider than long) traits. It is further characterized by an autapomorphic square cervical scale. Phylogenetic analysis assuming parsimony recovers B. morrinensis in a polytomy with B. variolosa and B. gaffneyi, outside the clade formed by the upper Maastrichtian forms B. sinuosa and B. praeclara. The holotype of Basilemys morrinensis provides the first evidence that this genus reached large size (~1 m long) in the Horseshoe Canyon Formation and was not diminutive as previously thought based on less complete shell material. Although Basilemys is usually regarded as terrestrial in habit based on its skull and limb morphology, we note that the low profile of its shell is a derived feature usually indicative of an aquatic mode of life. This suggests that there is yet much to learn about the life habits of this interesting turtle.