|— Subregion —|
|• Total||36,746 km2 (14,188 sq mi)|
|• Density||94/km2 ( 240/sq mi)|
|Time zone||EST (UTC−5)|
|• Summer (DST)||EDT (UTC−4)|
|Postal code prefixes||N|
|Area code(s)||519, 226, 905, 289|
Southwestern Ontario is a subregion of Southern Ontario in the Canadian province of Ontario, centred on the city of London. It extends north to south from the Bruce Peninsula on Lake Huron to the Lake Erie shoreline, and east to south-west roughly from Guelph to Windsor. The region had a population of 2,443,484 in 2006. Other significant towns and cities in the region are Brantford, Cambridge, Chatham, Goderich, Ingersoll, Kitchener, Owen Sound, Sarnia, St. Thomas, Stratford, Tillsonburg, Waterloo, Windsor, and Woodstock.
The Bruce Peninsula and Georgian Bay shoreline, including the Blue Mountains are also part of the Georgian Triangle, while regions north of London (Wingham, Mount Forest, Owen Sound, Hanover...) are considered the subregion of "Midwestern Ontario".
Southwestern Ontario occupies most of the Eastern Peninsula, bound on three sides by water: 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.
Most definitions of Southwestern or Western Ontario reach eastward to include Guelph and Orangeville, though recently these municipalities are also included in on the western side of the growing Golden Horseshoe region and many of their residents commute into the Greater Toronto Area.
Southwestern Ontario was first settled by Europeans in the early 18th century, when it was part of the Royal Province of New France. One of the oldest continuous settlement 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.
The region may also be referred to as Western Ontario, particularly in the names of institutions such as the University of Western Ontario. This term is falling into disuse, however, as the region is no longer the westernmost portion of the province. Western Ontario may also designate all the counties of southwestern Ontario except Essex, Kent, and Lambton—that is, the region of which London, Ontario is the central city.
Southwestern Ontario is a prosperous agricultural region whose chief crops are tobacco, sweetcorn, soybean, winter wheat, canola, and tomatoes. 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 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.
Weather and Climate 
Like other parts of southern Canada, the region brings warm or hot summers with normal thunderstorm occurrences. Some of these storms are severe, with damaging winds, hail and tornadoes all possible during peak season, May through September. The most likely areas for these kinds of weather events is within the Windsor - London corridor and north up to about Huron County. Winters are cold with less snowfall in the south towards Essex County and higher amounts north towards Bruce County.
List of counties or divisions 
Single-tier municipalities 
Regional municipalities 
- Bruce County
- Elgin County
- Essex County
- Grey County
- Huron County
- Lambton County
- Middlesex County
- Perth County
- Wellington County