Thornhill is a community in the Greater Toronto Area of Southern Ontario, Canada, located on the northern border of the city of Toronto. Once a municipal village, Thornhill is now a community and postal designation geographically split into two municipalities along Yonge Street, the city of Vaughan to the west and the city of Markham to the east. It is immediately south of Richmond Hill.

Thornhill was founded in 1794. Its first settlers on Yonge Street in Thornhill were Asa Johnson (who settled on the Vaughan side) and Nicholas Miller (who settled on the Markham side. Thornhill's location along Yonge Street, a major transportation route, proved to be beneficial to the community's growth throughout much of the twentieth century. in 1971, York Region was created, part of a wave of municipal re-organization which converted many townships into towns and eliminated many of the municipal forms of organization which had existed within those townships. The establishment of a regional administration effectively eliminated the Police Village of Thornhill. Thornhill's administration reverted to the newly formed towns of Markham and Vaughan at this time.

Woodbridge, Maple, and Unionville – as is typical for most urbanized former Greater Toronto Area (GTA) villages outside of the City of Toronto – Thornhill remained the postal designation for addresses within the former village's boundaries, and community organizations such as local newspapers, sports teams, and schools continued to operate under the Thornhill name and designation.

While the old village of Thornhill revolved around Yonge Street between Centre and John Streets, the community is typically thought to be located between Dufferin Street to the west, Highway 7 to the north, Steeles Avenue to the south, and Highway 404 to the east. A minor February 2008 snowstorm in Thornhill.Thornhill's growth since the 1960s and 1970s has been largely connected to its location bordering what is now the City of Toronto. Growth has continued apace. For instance, an "uptown" village denoted as Thornhill Town Centre, was developed at the north west corner of Bathurst and Centre Street, forming a continuous line of development north from the Promenade Shopping Centre also near Bathurst and Clark, whose massive parking lot is dotted by an increasing number of condominium buildings and stand-alone retail plazas.

The territory thought to constitute Thornhill has recently begun to expand northward in Thornhill-Vaughan, particularly west of Bathurst, due to the marketing initiatives of residential developers. A new area called "Thornhill Woods" was developed between Highway 7, Rutherford, Bathurst, and Dufferin. Another planned development, "Upper Thornhill Estates", extends well to the north of Major Mackenzie. In 2008 the City of Vaughan held a groundbreaking ceremony for the North Thornhill Community Centre, a new community centre to serve this population which is slated to open in 2010.

However, it should be noted that the area being labelled as "Upper" or "North" Thornhill is more traditionally part of Maple, another former village converted to a community when the Township of Vaughan was converted to a town. Thornhill has a very ethnically diverse population. It is home to significant visible minorities communities, but is particularly known for its large Jewish community: 36.6% of the population of Thornhill is Jewish, the highest in Canada. It is home to numerous Jewish synagogues and schools, particularly Ashkenazi and Sephardic Orthodox Jewish synagogues such as the Thornhill Community Shul, Westmount Shul and Learning Centre, the Beth Avraham Yoseph of Toronto, Sephardic Kehilla Centre, and Chabad centres such as the Jewish Russian Community Centre, as well as Conservative synagogues such as Beit Rayim as well as Reform Jewish institutions such as Temple Har Zion, Temple Kol Ami, and the Leo Baeck Day School, a major Reform Jewish primary school.