The distribution of the slipper limpet Crepidula fornicata (Linnaeus, 1758) in the northwestern Atlantic is generally understood to extend only as far north as Baie des Chaleurs (∼48°N, 66^°W) in the southwestern Gulf of St. Lawrence and only as far east as Cape Breton Island, Nova Scotia (-46^°N, 60^°W). Here we present evidence that this species is found considerably further north (to 51°N) and east (to 55^°W) than previously recognized. More specifically, its distribution is now known to include the Quebec “Lower North Shore” of the Gulf of St. Lawrence as well as the southern and western coasts of Newfoundland. Our evidence is based upon (1) shell collections from 12 museums in Canada and the U.S., (2) reports from both the primary and the secondary literature dating back to 1841, and (3) collections of C. fornicata in Newfoundland in 2009. While our investigations suggest that C. fornicata is expanding its geographic range in eastern Canada, we cannot be certain of this since the northern Gulf of St. Lawrence, Newfoundland, and Labrador have not been extensively sampled in the past for non-commercial benthic marine molluscs. Factors such as larval drift in known surface currents may be sufficient to explain such supposed range extensions. Clarification of current geographic ranges of marine species, particularly of well-studied and easily recognizable taxa such as C. fornicata, remains an important task for establishing temporal benchmarks against which to assess the effect of climate change on the marine environment.