South Dundas is a municipality in eastern Ontario, Canada, in the United Counties of Stormont, Dundas and Glengarry along the north shore of the St. Lawrence River. It is located approximately 100 kilometres south of Ottawa and is midway between Kingston and Montreal, Quebec.

The United Counties of Stormont, Dundas and Glengarry (SDG) is an upper-tier municipality in the Canadian province of Ontario that comprises three historical counties and excludes the City of Cornwall and the Mohawk Nation of Akwesasne. However, both Cornwall and Akwesasne form part of a larger census division named for the United Counties of Stormont, Dundas and Glengarry. The municipality's administrative office is located within Cornwall.