File:Junction of Ontario Highway 427 and Gardiner Expressway, Toronto, Ontario (29358645403).jpg

King's Highway 427 (pronounced "four twenty-seven"), also known as Highway 427 and colloquially as the 427, is a 400-series highway in the Canadian province of Ontario that connects the Queen Elizabeth Way (QEW) and Gardiner Expressway in Toronto to Highway 401 and with York Regional Road 7 (formerly Highway 7) further north in the City of Vaughan. An arterial extension, known as York Regional Road 99, continues 800 metres (2,600 ft) north to Zenway Boulevard. It is Ontario's second busiest freeway by volume and 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.

Highway 427 is the main feeder to Toronto Pearson International Airport from the north and south. However, while much of the traffic from Highway 407, Highway 401 (eastbound), and the QEW/Gardiner Expressway makes use of the freeway for airport access, it also serves as a major traffic route for the western portion of Toronto (Etobicoke), the northeastern portion of Mississauga (Malton), the southeastern portion of Brampton (Claireville), and the western portion of Vaughan (Woodbridge). The section between Highway 401 and Dundas Street is a heavily traversed transit corridor; the 1.61-kilometre (1.00 mi) stretch between Burnhamthorpe and Rathburn saw an average of over 400,000 vehicles and over 5,000 buses per day in 2016. GO Transit, MiWay, and the Toronto Transit Commission (TTC) all operate express buses along this section of the highway.

First designated in 1972, Highway 427 assumed the recently-completed 12-lane collector-express freeway of Highway 27, as well as a short freeway north of Highway 401 known as the Airport Expressway. Both routes were upgraded throughout the 1950s and 1960s, eventually becoming intertwined into the present configuration in 1972. The freeway was extended north from Pearson Airport to Highway 7 over the following twenty years. A short arterial extension was later built past Highway 7 to Zenway Boulevard, though this section is designated as York Regional Road 99. Plans have been announced to extend Highway 427 north to Major Mackenzie Drive; construction began in 2017 and is expected to be completed by 2020.

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