St. Mary's, officially named the Municipality of the District of St. Mary's, is a district municipality in Guysborough County, Nova Scotia, Canada.
The district municipality occupies the western half of the county and its administrative seat is in the village of Sherbrooke.
The first inhabitants of the area were the indigenous Mi'kmaq people. The French established a presence along the St. Mary's River in the 17th century. The area was widely settled by the British in the early 19th century.
The District of St.Mary's was created in 1879 with the act establishing municipal governments in Nova Scotia. Prior to this, the tasks of local administration were combined with local courts and the Justices of the Peace.
In the 2021 Census of Population conducted by Statistics Canada, the Municipality of the District of St. Mary's had a population of living in of its total private dwellings, a change of from its 2016 population of . With a land area of , it had a population density of in 2021.
The district municipality is divided into five electoral districts for municipal representation.