The Regional Municipality of York, also called York Region, is a regional municipality in Southern Ontario, Canada, between Lake Simcoe and Toronto. The region was established after the passing of then Bill 102, An Act to Establish The Regional Municipality of York, in 1970. It replaced the former York County in 1971, and is part of the Greater Toronto Area and the inner ring of the Golden Horseshoe. The regional government is headquartered in Newmarket.

As of the 2021 census, York Region's population was 1,173,334, with a growth rate of 5.7% from 2016. The Government of Ontario expects its population to surpass 1.5 million residents by 2031. The three largest cities in York Region are Markham, Vaughan and Richmond Hill.

At a meeting in Richmond Hill on May 6, 1970, officials representing the municipalities of York County approved plans for the creation of a regional government entity to replace York County. The plan had been presented in 1969 by Darcy McKeough, the Ontario Minister of Municipal Affairs, taking about a year to determine municipal boundaries within the new regional government.

The Regional Municipality of York was created by Act of the Legislative Assembly of Ontario in 1970 (Bill 102 An Act to Establish The Regional Municipality of York), which took effect on January 1, 1971. The creation of the regional municipality resulted in the consolidation of the fourteen former municipalities of York County into nine new municipalities:

The township of Whitchurch merged with the town of Stouffville to create the town of Whitchurch–Stouffville, ceding land to Aurora, Newmarket, and Richmond Hill to the west of the proposed Highway 404 and annexing a northern strip of land from the township of Markham. The western boundary of the new town of Markham was defined to be at Yonge Street, where its northern boundary was formed with Richmond Hill (to which it ceded land) and its western boundary with the new town Vaughan. The new town of Vaughan would consist of all communities in the area bounded by Markham and Richmond Hill in the east, Metro Toronto in the south, the periphery of the regional municipality in the west, and the new township of King in the north.

The townships of Georgina, North Gwillimbury, and Sutton were merged into the township of Georgina, and the East Gwillimbury neighbourhood of East Gwillimbury Heights was merged into Newmarket. King formed the northwestern part of the new region, but the eastern lot from Bathurst Street to Yonge Street was ceded to Newmarket, Aurora, and Oak Ridges, the last of which became a part of Richmond Hill. The boundary between Aurora and Newmarket was defined to be St. John's Sideroad, and Newmarket's northern boundary was defined to be Green Lane.

The towns of Aurora, Newmarket, and Richmond Hill were defined to be the growth centres for the regional municipality, which was to become a greenbelt between the denser urban areas of Toronto to the south and Barrie to the north. The growth centres were each restricted to grow to a maximum population of 25,000 by 2000, and the regional municipality to 300,000.

The municipal realignment merged 40% of East Gwillimbury's population into Newmarket. The council of East Gwillimbury voted to amalgamate with Newmarket, but Newmarket council opposed the amalgamation. In the plan presented by McKeough, the councils of the towns of Newmarket and Aurora were given ten years to decide whether or not to amalgamate.