King's Highway 403, or simply Highway 403, is a 400-series highway in the Canadian province of Ontario that travels between Woodstock and Mississauga, branching off from and reuniting with Highway 401 at both ends and travelling south of it through Hamilton and Mississauga. It is concurrent with the Queen Elizabeth Way (QEW) for 22 km (14 mi) from Burlington to Oakville. The Highway 403 designation was first applied in 1963 to a short stub of freeway branching off the QEW, and the entire route was completed on August 15, 1997, when the section from Brantford to the then-still independent Town of Ancaster was opened to traffic. The section of Highway 403 between Woodstock and Burlington was formally dedicated as the Alexander Graham Bell Parkway on April 27, 2016, in honour of Alexander Graham Bell.

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.