Thunder Bay District is a district and census division in Northwestern Ontario in the Canadian province of Ontario.

Thunder Bay District was created in 1871 by provincial statute from the western half of Algoma District, named after a large bay on the north shore of Lake Superior.

Its northern and western boundaries were uncertain until Ontario's right to Northwestern Ontario was determined by the Judicial Committee of the Privy Council.

[4] Until about 1902 it was often called Algoma West from the name of the provincial constituency established in 1885.

The following districts include areas that were formerly part of Thunder Bay District: As a census division in the 2021 Census of Population conducted by Statistics Canada, the Thunder Bay District had a population of 146,862 living in 64,601 of its 72,510 total private dwellings, a change of 0.6% from its 2016 population of 146,048.