King's Highway 420, commonly referred to as Highway 420, is a 400-series highway in the Canadian province of Ontario that connects the Queen Elizabeth Way (QEW) with downtown Niagara Falls. It continues east as a limited-access expressway named Niagara Regional Road 420 to connect with the Rainbow Bridge international crossing between Canada and the United States over the Niagara River; this was part of Highway 420 until 1998. West of the QEW, the freeway ends at an at-grade intersection with Montrose Road. The highway has a speed limit of 80 kilometres per hour (50 mph), making it the only 400-series highway to have a speed limit less than 100 kilometres per hour (62 mph) for its entirety.

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.