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

In 2016, the population was 146,048. The land area is 103,719.51 square kilometres (40,046.33 sq mi); the population density was 1.4 per square kilometre (3.6/sq mi).^[1] Most of the district (93.5%) is unincorporated and part of the Unorganized Thunder Bay District.

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.^[5] 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:

• Rainy River, created in 1885
• Kenora, created in 1907 from Rainy River District
• Cochrane, created in 1921

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. With a land area of 102,895.48 km² (39,728.17 sq mi), it had a population density of 1.4/km² (3.7/sq mi) in 2021.^[7]

• Arthur, Elizabeth. Thunder Bay District, 1821-1892: A Collection of Documents. Toronto: Champlain Society Publications, 1973.

Wikimedia Commons has media related to Thunder Bay District, Ontario.

• Edward Island (Lake Superior)
• Esnagami Lake
• List of Ontario Census Divisions
• List of townships in Ontario
• List of secondary schools in Ontario#Thunder Bay District