Southwestern Ontario is a secondary region of Southern Ontario in the Canadian province of Ontario. It occupies most of the Ontario Peninsula bounded by Lake Huron, including Georgian Bay, to the north and northwest; the St. Clair River, Lake St. Clair, and Detroit River, to the west; and Lake Erie to the south. To the east, on land, Southwestern Ontario is bounded by Central Ontario and the Golden Horseshoe. The region had a population of 2,796,367 in 2021. It is sometimes further divided into "Midwestern Ontario" covering the eastern half of the area and the heart of Southwestern Ontario encompassing the western half of the region.

Quick Facts Country, Province ...

Southwestern Ontario
Secondary region
Coordinates: 43°30′N 81°00′W
Country	Canada
Province	Ontario
Area
• Total	36,797.54 km2 (14,207.61 sq mi)
Population  (2021)
• Total	2,796,367
• Density	76/km2 (200/sq mi)
Time zone	UTC−5 (EST)
• Summer (DST)	UTC−4 (EDT)
Postal code prefixes	N
Area code(s)	519, 226, 548, 905, 289, 365

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The Government of Ontario also classifies municipalities along the eastern side of Southwestern Ontario near the Grand River, including Wellington County (containing Guelph), the Region of Waterloo (containing Kitchener, Waterloo, and Cambridge), and Brant County (containing Brantford), as part the "Greater Golden Horseshoe" region that surrounds western Lake Ontario.^[1] Some non-profit organizations, government bodies, and news organizations also classify a larger swath of the eastern side of Southwestern Ontario (the area from Huron County, Bruce County, and Grey County in the north, southward through Dufferin County, Wellington County, the Region of Waterloo, Perth County, and Oxford County, and sometimes south to Brant County and Norfolk County on Lake Erie) as being "Midwestern Ontario."^{[2][3][4][5][6][7][8][9][10][11][excessive citations]} This thereby places only the counties southwest of this in their definition of Southwestern Ontario,^[12] although in some cases Perth County is also grouped in Southwestern Ontario.^{[citation needed]}

Archaeological findings from the Princess Point Complex suggest that indigenous peoples grew maize in the region as early as 260 CE. Iroquoians are recognizable from 500 CE, and by 900 CE longhouse villages began appearing near modern-day Brantford.^[13]

Europeans settlement began in the early 18th century, when it was part of the Royal Province of New France. One of the oldest continuous settlements in the region is Windsor, which originated as a southerly extension of the settlement of Fort Detroit in 1701. With the transfer of New France to British control in 1763, the region was part of the British Province of Quebec, 1774 to 1791; the Province of Upper Canada, 1791 to 1841; and the Canada West division of the Province of United Canada, 1841 to Confederation in 1867, when United Canada was formally partitioned into the provinces of Ontario and Quebec.

During the 19th century and early 20th century, the largest city in Southwestern Ontario was Windsor; however, as both cities grew, Windsor was outpaced by the faster growth of London, and passed the mantle of regional anchor to that city in the 1960s. Late in the 20th century the Kitchener–Cambridge–Waterloo metropolitan area became the most populous metropolitan area in southwestern Ontario surpassing the London-St. Thomas metropolitan area, and serves as the anchor of Midwestern Ontario.

Southwestern Ontario is a prosperous agricultural region whose chief crops are tobacco, sweet corn, soybean, winter wheat, canola, and tomatoes.^[14] Additionally, Southwestern Ontario has developed a significant concentration of commercial greenhouses, predominantly centred in the town of Leamington, where tomatoes, peppers, cucumbers, flowers, and cannabis are grown. It is the largest concentration of commercial greenhouses in North America. Dairy and beef farming, breeding and training of standardbred horses and wine growing and production are also important industries. Its climate is among the mildest in Canada. Although brief periods of winter can be severe, summers are hot and humid with a longer growing season than in most of the country.

A large section of Southwestern Ontario was part of the Talbot Settlement, and the region has benefited from the settlement’s facilitation of agriculture and of trade in general. Its economy is heavily tied in with that of the midwestern United States, in particular the border state of Michigan. Auto manufacturing and parts, agriculture and hi-tech industries are key components of the region’s economy. The region also provides important transportation routes for commercial trucking, railway and tanker shipping from Detroit-Windsor and Port Huron, Michigan-Sarnia linking Canada with major markets in the eastern and midwestern United States.

Southwestern Ontario is located within the Mixedwood Plains Ecozone (as classified by Environment and Climate Change Canada, further subclassified into the Lake Erie Lowland and Manitoulin-Lake Simcoe regions),^[18] which largely consists of glacial till plains underlain by sedimentary limestone bedrock. The vast majority of Southwestern Ontario maintains a relatively flat geography with rolling hills, with the exception of areas near the Niagara Escarpment in Bruce and Grey Counties, where exposed limestone cliff faces can be seen along the shores of Georgian Bay. Sandier soils are generally located near Lake Erie, resulting in the creation of a sandy escarpment along the entire north shore, in addition to many sandspit peninsulas, including Point Pelee in Essex County, Pointe aux Pins (Rondeau) in Chatham-Kent, and Long Point in Norfolk County. Pelee Island, the southernmost populated area in Canada, is located south of Point Pelee in Lake Erie.

Southwestern Ontario was originally covered by vast forests prior to agricultural development in the region. Two forest regions are located in Southwestern Ontario;^[19] Carolinian forest, located predominantly along Lake Erie and the southern tip of Lake Huron,^[20] and Great Lakes-St. Lawrence forest, located around Lake Huron and Georgian Bay.^[21] A very small percentage of forest coverage areas remain in the region due to extensive agricultural development and urbanization, but conservation efforts are underway to preserve and/or protect these forests. Three major federally protected areas, Point Pelee National Park, Bruce Peninsula National Park, and Long Point National Wildlife Area, are located in Southwestern Ontario. In 2021, the federal government announced plans to designate a new National Urban Park in Windsor's Ojibway Prairie Complex.^[22]

The Carolinian forest zone is Canada's smallest forest zone, but it is home to a very high biodiversity of species due to it being one of the warmest regions in the country. Over 500 rare and/or endangered species are located in this region^[20] and is estimated to contain around 25% of Canada's species at risk, including the American badger, Midland painted turtle, Jefferson salamander, monarch butterfly, and southern flying squirrel.^[23]

Southwestern Ontario's climate is heavily influenced by the proximity of the Great Lakes. Winters in the region are typically milder than the rest of Ontario due to the gradual release of stored heat in the lakes. Despite this, however, portions of the region experience significantly more snow than others due to lake-effect snow caused by eastward winds. A portion of the regional Snowbelt is located in Bruce, Grey, Huron, and Middlesex Counties.^[24] In a sharp contrast, Essex County and Chatham-Kent receive less snow than average due to their location further south.^[25] Summers are typically hot and humid, with Windsor experiencing the warmest weather in Ontario. Summers are often cooler and less humid on the peninsulas in Lake Erie and Lake Huron. Southwestern Ontario also experiences the highest annual frequency of thunderstorms in Canada, with Windsor, Chatham-Kent, and London experiencing the most days on average with lightning per year.^[26] Southwestern Ontario also regularly experiences severe weather events, including tornadoes, due to its proximity to the Great Lakes and warmer-than-average weather during the summer.

Under the Köppen climate classification, the vast majority of Southwestern Ontario has a humid continental climate, with the vast majority of the area experiencing a warm-summer Dfb climate, with the exception of Windsor and Chatham-Kent, which experiences a hot-summer Dfa climate. Additionally, a very small area near Long Point experiences a temperate oceanic Cfb climate, due to its location in Lake Erie creating slightly cooler summers and slightly warmer winters than the remainder of the province.