King's Highway 409, commonly referred to as Highway 409 and historically as the Belfield Expressway, is a 400-series highway in the Canadian province of Ontario that extends from Highway 401 in Toronto to Pearson International Airport, west of Highway 427, in Mississauga. It is a short freeway used mainly as a spur route for traffic travelling to the airport or Highway 427 northbound from Highway 401 westbound (and vice versa), as these route movements are not accommodated at the complex interchange between Highways 401 and 427.

Planning for Highway 409 took place in the late 1960s amidst considerable controversy around its originally proposed path through the historic town of Malton. Eventually, the route was changed to provide access to the airport instead of northward towards Brampton and was completed through the mid-1970s, opening in 1978. The significance of the route has increased over the years alongside expansion of the airport. In 2000, the Greater Toronto Airports Authority (GTAA) purchased the section of the highway west of Highway 427 in order to modify the ramps leading into the airport.

The speed limit along Highway 409 is 100 kilometres per hour (62 mph) east of Highway 427 and 80 km/h (50 mph) west of it. It is patrolled by the Ontario Provincial Police east of the Toronto–Peel boundary and by the Peel Regional Police to the west of it. The original name of the freeway was derived from Belfield Road, which runs parallel to and north of the highway in Toronto from Kipling Avenue to Atwell Drive.