Northwestern Ontario is the region within the Canadian province of Ontario which lies north and west of Lake Superior, and west of Hudson Bay and James Bay. It includes most of subarctic Ontario. Its western boundary is the Canadian province of Manitoba, which disputed Ontario's claim to the western part of the region. Ontario's right to Northwestern Ontario was determined by the Judicial Committee of the Privy Council in 1884 and confirmed by the Canada (Ontario Boundary) Act, 1889 of the Parliament of the United Kingdom. In 1912 the Parliament of Canada by the Ontario Boundaries Extension Act gave jurisdiction over the District of Patricia to Ontario, thereby extending the northern boundary of the province to Hudson Bay.

For some purposes, Northwestern Ontario and Northeastern Ontario are treated as separate regions, while for other purposes they are grouped together as Northern Ontario.