Dufferin County is a county and census division located in Central Ontario, Canada. The county seat is Orangeville, and the current Warden is Janet Horner. The current chief administrative officer is Sonya Pritchard. Dufferin covers an area of 1,486.77 square kilometres (574.05 sq mi), and its population was 66,257 at the time of the 2021 Census.

It was originally organized as the "Provisional County of Dufferin", with preparatory work authorized by the Legislative Assembly of Ontario in 1875 and the actual formation taking effect in 1881, being created from parts of the counties of Grey and Simcoe, on the north and east, and from the County of Wellington on the south and west.

The Village of Grand Valley was erected from East Luther in 1897, and the two municipalities amalgamated in 1995 to form the Township of East Luther Grand Valley, which was erected into the Town of Grand Valley in 2012.

The county gets its name from Frederick Hamilton-Temple-Blackwood, 1st Marquess of Dufferin and Ava, who was Governor General of Canada between 1872-1878. Originally an agriculturally based economy, Dufferin's economy has diversified to include commercial and retail businesses, industries related to residential and commercial construction (building, supplies, aggregates, real estate) and manufacturing. A portion of Dufferin’s economy still depends on agriculture but tourism is becoming more important as the county takes a more positive role in attracting visitors.

Dufferin County is the highest plateau immediately south of Georgian Bay, and as such forms the watershed divide between the four lakes: Huron, Erie, Ontario and Simcoe. Four rivers — Saugeen, Grand, Credit and Nottawasaga — take their rise in Dufferin or in adjacent areas nearby and drain through the county.

The county is a lofty table-land that is about 518 metres (1,700 ft) above sea-level and about 427 metres (1,400 ft) above the level of downtown Toronto, as a result of being mostly situated in the Niagara Escarpment, although the highest peaks are still lower than the Blue Mountains north of Dufferin. The County of Dufferin, sits on the fringe of the Greater Toronto Area, about 100 kilometres (62 mi) northwest of Toronto. It is largely a rural county that contains three urban-centre towns; Grand Valley, Shelburne, and Orangeville. Orangeville, the county seat, is situated on the southern border of the county and is the largest urban centre, with just over half the population.

As a census division in the 2021 Census of Population conducted by Statistics Canada, Dufferin County had a population of 66,257 living in 23,310 of its 24,388 total private dwellings, a change of 7.3% from its 2016 population of 61,735. With a land area of 1,486.77 km² (574.05 sq mi), it had a population density of 44.6/km² (115.4/sq mi) in 2021.

Upper Grand District School Board operates secular Anglophone public schools. The Dufferin-Peel Catholic District School Board operates Anglophone Catholic public schools. The Conseil scolaire Viamonde operates secular Francophone schools serving the area. The Conseil scolaire de district catholique Centre-Sud (CSDCCS) Archived 1999-10-11 at the Wayback Machine operates Catholic Francophone schools serving the area.