A significant and recruiting population of Obovaria olivaria (Hickorynut Mussel) was confirmed in the Mississagi River, Lake Huron drainage, ON, Canada. This large river unionid mussel is known to use Acipenser fulvescens (Lake Sturgeon) and Scaphirhynchus platorynchus (Shovelnose Sturgeon) as hosts. The Mississagi River is known to have a spawning Lake Sturgeon population. In 9.1 person-hours of snorkel and SCUBA searches, 10 live O. olivaria ranging in length from 36 to 79 mm, including six gravid females, were collected in sandy substrates with water depths from 1.5 to 4 m. Obovaria olivaria in the Mississagi River represent a significant range disjunction from the other extant populations in Canada in the Ottawa and St. Lawrence river drainages. As the conservation status of this rare mussel in Canada is assessed, the geographic genetic population structure and spatial extent among known areas should be studied to understand the post-glacial redistribution of the species.