King's Highway 404, also known as Highway 404 and colloquially as the 404, is a 400-series highway in the Canadian province of Ontario. A continuation of the municipal Don Valley Parkway (DVP) north of Highway 401, it connects Toronto with East Gwillimbury. The 50.1-kilometre (31.1 mi) controlled-access freeway also connects with Highway 407 in Markham, which formed the northeastern ring road of the Greater Toronto Area until the opening of Highway 412 in 2016. Highway 404 provides access to the eastern edge of Richmond Hill, Aurora and Newmarket and the western edge of Whitchurch-Stouffville, in addition to the southern edge of Keswick.

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.