South Glengarry is a township in eastern Ontario, Canada on the Saint Lawrence River in the United Counties of Stormont, Dundas and Glengarry.

The township was established on January 1, 1998, with the amalgamation of the former Townships of Charlottenburgh and Lancaster, along with the Village of Lancaster.

The township of South Glengarry comprises a number of villages and hamlets, including the following communities:

The township administrative offices are located in Lancaster.

Charlottenburgh and Lancaster were two of the original eight "Royal Townships", established along the Saint Lawrence River in Upper Canada in the 1780s. This area was first settled by United Empire Loyalists. The development of this area was encouraged by Sir John Johnson, then the Superintendent General and Inspector General of Indian Affairs, who had been forced to abandon his land holdings in New York State during the American Revolution. ^[25]

From the late 18th century to the early 19th century, the area was almost entirely settled by Scottish highlanders, especially from Inverness-shire, after the Highland Clearances. Canadian Gaelic / Scottish Gaelic had been spoken in Glengarry County since its first settlement in 1784.

Sir John built a house in Williamstown which was completed in 1785 and is now a Canadian National Historic Site, the Sir John Johnson Manor House. A grist mill and saw mill, now gone, were also built on the same location. Williamstown also has the oldest house in Ontario, The Bethune-Thompson House built in 1784. Occupants over the years have included the Reverend John Bethune (1751–1815), the great-great-grandfather of Doctor Norman Bethune, and David Thompson, Canadian explorer.

Some of the main partners of the North West Company, including Hugh McGillis, lived in this area.

Alexander McMartin, the first person born in Upper Canada to serve in the Legislative Assembly, was from Martintown.

The Glengarry Celtic Music Hall of Fame is located in Williamstown.

Williamstown is also home to Ontario’s oldest continuing annual fair, which celebrated its bicentennial in 2012.

(North America’s oldest being the Hants County agricultural exhibition of Windsor, Nova Scotia, some 47 years older, est. 1765)

The Nor'Westers and Loyalist Museum is also located in Williamstown.

South Glengarry is the location of four National Historic Sites of Canada : the Bethune-Thompson House, ^[27] the Glengarry Cairn, ^[29] the Sir John Johnson House, ^[31] and the ruins of St. Raphael's Roman Catholic Church. ^[33]

St. Raphael's Catholic Church was built commencing 1821 under the authority of Alexander Macdonell later Bishop of Regiopolis (now Kingston, Ontario). This is one of the oldest churches in what was then the colony of Upper Canada. In late 1970, the church interiors, roof and tower were destroyed by fire, but the ruins were preserved. In 1973, a smaller church with the same name was built, attached to the ruins.

The Char-Lan Rebels of the CCHL2 League play out of the Char-Lan Recreation Centre in Williamstown.

Canadian author Hugh Hood mentions Williamstown in his short story "Getting to Williamstown," first published in 1928.