Southwestern Ontario (census population 2,796,367 in 2021) 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. It borders the United States via Michigan.

Southwestern Ontario is often not consistently defined. In certain documents, the Government of Ontario classifies municipalities along the eastern side of Southwestern Ontario near the Grand River, including Brant County, Waterloo Region, and Wellington County, as part of the Greater Golden Horseshoe region that surrounds western Lake Ontario, mainly due to the presence of modern transportation connections that link these areas to the core sections of the Golden Horseshoe. A more traditional definition of the region's boundary can be traced back to early colonial districts in the British Province of Quebec and Upper Canada. The Western District, originally known as the Hesse District from 1788 to 1792, was originally designated as everything west of a north-south line intersecting the extreme projection of Long Point into Lake Erie, which roughly follows the eastern boundaries of modern Brant, Grey, Dufferin, Norfolk, and Wellington Counties.

The northern portion of Southwestern Ontario is sometimes referred to as Midwestern Ontario. This area includes Bruce, Grey, Dufferin, Huron, Perth, and Wellington Counties, corresponding roughly with the historical boundaries of Queen's Bush, an area of crown land that did not see formal subdivision or wide-scale settlement until the 1830s.

Indigenous peoples had occupied Southwestern Ontario for thousands of years prior to European settlement. Archaeological sites such as the Princess Point Complex and the Parkhill Complex indicate the presence of Paleo-Indians in the area dating back approximately 11,000 years. First Nations peoples located in the region included Anishinaabe, Algonquin, Haudenosaunee (Iroquois), and Wendat peoples.

Initial European settlement and colonization of the region occurred in the 17th and 18th centuries by the French as part of the colony of Canada within New France. The oldest continually inhabited European settlement in Southwestern Ontario is Windsor, which originated as a southerly extension of the settlement of Fort Detroit in 1701 and was established as la Petite Côte in 1749. Further development occurred under the British as part of the Province of Quebec from 1763 to 1791, and eventually the Province of Upper Canada from 1791 to 1841. One of the earliest British settlements, Port Talbot, was settled by Thomas Talbot in 1803, and was amongst the first successful settlements due to the construction of Talbot Trail, one of the earliest major roads in the region. Kitchener, originally known as Ebytown and later Berlin, was established by German Mennonite and Pennsylvania Dutch settlers in 1807, after sections of the Haldimand Tract were purchased from the Six Nations in 1798. London, originally called Georgiana upon its selection in 1793 as the potential site of the future capital of Upper Canada, was formally founded in 1826 by John Graves Simcoe, the first Lieutenant Governor of Upper Canada. Guelph was founded in 1827 by John Galt as a headquarters for the Canada Company. Many other cities in the region did not see major settlement until the mid-19th century, when agricultural expansion was occurring. During the early-to-mid 19th century, the region was the northern terminus of the Underground Railroad for enslaved African Americans seeking escape from the United States. Thousands of slaves escaped to the region, settling and owning farms in areas such as Essex County, Chatham-Kent, and Queen's Bush. The region's economy was predominantly focused on agriculture until the late 19th century, when industrialization of the region began to occur. Southwestern Ontario emerged as an ideal location for manufacturing due to the presence of cheap and abundant hydroelectric power sourced from the nearby Niagara Falls, and eventually became Canada's manufacturing heartland, which attracted multiple heavy industrial sectors, including automotive manufacturing, chemical production, petrochemical refinement and transport, and food processing. Automotive manufacturing is central to the regional economy, with major active automotive plants being located in Windsor, London, Ingersoll, Woodstock, and Cambridge. The effects of the 2008–2010 automotive industry crisis on Canada included a large number of plants shutting down across the region in the following years. The 2020s saw a widespread resurgence in industrial development, predominantly due to the comparative lack of available land for development and fast-rising property prices in the Golden Horseshoe. Multiple large investments in electric vehicle production were also made in the region in the 2020s, and resulted in the establishment of multiple plants for battery production in Windsor and St. Thomas.

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), which largely consists of glacial till and glaciolacustrine sand/silt/clay plains underlain by sedimentary limestone, dolostone, siltstone, sandstone, and shale bedrock, with pockets of evaporite gypsum and salt beds. Bedrock is rarely exposed in the region, with the exception being the Bruce Peninsula. 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; Carolinian forest, located predominantly along Lake Erie and the southern tip of Lake Huron, and Great Lakes-St. Lawrence forest, located around Lake Huron and Georgian Bay. 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.

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 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.