| Official Name: | Thunder Bay District |
| Settlement Type: | District |
| Coordinates: | 50°N -88°W |
| Subdivision Type: | Country |
| Subdivision Name: | Canada |
| Subdivision Type1: | Province |
| Subdivision Type2: | Region |
| Subdivision Name1: | Ontario |
| Subdivision Name2: | Northwestern Ontario |
| Established Title: | Created |
| Established Date: | 1871 |
| Leader Title: | MPs |
| Leader Name: | Carol Hughes (NDP) Patty Hajdu (Liberal) Don Rusnak (Liberal) |
| Leader Title1: | MPPs |
| Leader Name1: | Michael Gravelle (OLP) Michael Mantha (NDP) Judith Monteith-Farrell (NDP) |
| Area Land Km2: | 103719.51 |
| Elevation Footnotes: | [1] |
| Elevation M: | 220 |
| Elevation Min M: | 183 |
| Population Total: | 146048 |
| Population As Of: | 2016 |
| Population Density Km2: | 1.4 |
| Postal Code Type: | Postal code span |
| Postal Code: | P0S, P0T, P7A to P7G, P7J to P7L |
| Area Code: | 807 |
| Timezone1: | Eastern |
| Utc Offset1: | -05:00 |
| Timezone1 Dst: | EDT |
| Utc Offset1 Dst: | -04:00 |
| Timezone1 Location: | East of 90° west |
| Timezone2: | Central |
| Utc Offset2: | -06:00 |
| Timezone2 Dst: | CDT |
| Utc Offset2 Dst: | -05:00 |
| Timezone2 Location: | West of 90° west |
| Elevation Max M: | 640 |
| Blank Name: | Largest communities [2] |
| Blank Info: | Thunder Bay (109,140) Oliver Paipoonge (5,757) Greenstone (4,906) |
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 ; the population density was . 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.[3] 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 living in of its total private dwellings, a change of from its 2016 population of . With a land area of 102895.48km2, it had a population density of in 2021.[4]