King's Highway 427, also known as Highway 427 and colloquially as the 427, is a 400-series highway in the Canadian province of Ontario that runs from the Queen Elizabeth Way (QEW) and Gardiner Expressway in Toronto to Major Mackenzie Drive in Vaughan. It is Ontario's second busiest freeway by volume and the third busiest in North America, behind Highway 401 and Interstate 405 in California. Like Highway 401, a portion of the route is divided into a collector-express system with twelve to fourteen continuous lanes. Notable about Highway 427 are its several multi-level interchanges; the junctions with the QEW/Gardiner Expressway and Highway 401 are two of the largest interchanges in Ontario and were constructed between 1967 and 1971, while the interchanges with Highway 409 and Highway 407 were completed in 1992 and 1995, respectively.

The 400-series highways are a network of controlled-access highways in the Canadian province of Ontario, forming a special subset of the provincial highway system. They are analogous to the Interstate Highway System in the United States or the Autoroute system of neighbouring Quebec, and are regulated by the Ministry of Transportation of Ontario (MTO). The 400-series designations were introduced in 1952, although Ontario had been constructing divided highways for two decades prior. Initially, only Highways 400, 401 and 402 were numbered; other designations followed in the subsequent decades. The network is situated almost entirely in Southern Ontario, although Highway 400 extends into the more remote northern portion of the province.