Thunder Bay is a city in and the seat of Thunder Bay District, Ontario, Canada. It is the most populous municipality in Northwestern Ontario and the second most populous (after Greater Sudbury) municipality in Northern Ontario. Its population is 108,843 according to the 2021 Canadian census. Located on Lake Superior, the census metropolitan area of Thunder Bay has a population of 123,258 and consists of the city of Thunder Bay, the municipalities of Oliver Paipoonge and Neebing, the townships of