Lancaster is an unincorporated community in South Glengarry, Ontario, Canada, located near the St. Lawrence River. It is recognized as a designated place by Statistics Canada.
On January 1, 1998, the Village of Lancaster, together with the Townships of Lancaster and Charlottenburgh, was amalgamated into the new Township of South Glengarry.
In the 2021 Census of Population conducted by Statistics Canada, Lancaster had a population of 729 living in 320 of its 342 total private dwellings, a change of from its 2016 population of 750. With a land area of , it had a population density of in 2021.