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 2011, the population was 146,057. 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). 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. 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:

Almost 80 per cent of the district's population lives in the city of Thunder Bay Census Metropolitan Area; no other community in the district exceeds 7,000 in population.

Thunder Bay is a city in and the seat of Thunder Bay District, Ontario, Canada. It is the most populous municipality in Northwestern Ontario with a population of 108,359 as of the Canada 2011 Census, and the second most populous in Northern Ontario after Greater Sudbury. The census metropolitan area of Thunder Bay has a population of 121,596, and consists of the city of Thunder Bay, the municipalities of Oliver Paipoonge and Neebing, the townships of Shuniah, Conmee, O'Connor and Gillies and the Fort William First Nation.

European settlement in the region began in the late 17th century with a French fur trading outpost on the banks of the Kaministiquia River. It grew into an important transportation hub with its port forming an important link in the shipping of grain and other products from western Canada through the Great Lakes and the Saint Lawrence Seaway to the east coast. Forestry and manufacturing played important roles in the city's economy. They have declined in recent years, but have been replaced by a "knowledge economy" based on medical research and education. Thunder Bay is the site of the Thunder Bay Regional Research Institute.