King's Highway 16, commonly referred to as Highway 16 and historically as the Prescott Highway, is a provincially maintained highway in the Canadian province of Ontario. The highway once travelled from near Prescott to Ottawa, traversing the distance between the St. Lawrence River and the Ottawa River. However, its length was truncated significantly when most of the route was twinned with a second roadway, and renumbered as Highway 416. A short stub remains through Johnstown, providing access to the Ogdensburg–Prescott International Bridge across the St. Lawrence River to Ogdensburg, New York, where New York State Route 812 continues south.

The Ottawa–Prescott Road, designated in 1918, was one of the first three highways designated in Ontario. By the time the route was numbered as Provincial Highway 16 in August 1925, it was mostly paved, except for portions south of Kemptville, which were paved by 1930. It immediately became the primary route between Toronto and Ottawa, via Highway 2, and as such saw many improvements and realignments carried out over the next three decades. In the 1960s, plans arose for a controlled-access highway to connect Highway 401 with Ottawa, which resulted in the construction of a complete realignment of Highway 16 north of Johnstown. This two-lane highway, known as Highway 16 New, was built between 1969 and 1983; enough land was purchased to build a second two-lane roadway to twin the highway. The twinned roadway was completed between 1989 and 1999, after which the route was renumbered with a 400-series designation.

Former portions of Highway 16 can be followed north from Johnstown, through Spencerville and Kemptville to the Rideau River along Leeds and Grenville County Road 44. Beyond the River it followed Ottawa Road 5 to North Gower, and thereafter Ottawa Road 73 (Prince of Wales Drive) into downtown Ottawa.

Early impetus for a route connecting Ottawa to the St. Lawrence River began with lobbying by automobile clubs in the early 1910s. The Prescott Highway was established as a provincial highway in 1918, shortly after The Provincial Highway (which would become Highway 2). The 92.7 km (57.6 mi) Ottawa–Prescott Highway was assumed by the Department of Public Highways (DPHO) on August 15. The new route was initially in an unfit condition for traffic. For example, the 1918 DPHO Annual Report noted that in North Gower Township, the road "was in places very narrow and the sides grown up with brush and small trees. The road surface was in very bad shape." Work began immediately to clear, widen, grade, and gravel the route, which was in many cases only 10 m (33 ft) wide between the overgrown fence lines. Paving began in 1922, starting at the Central Experimental Farm in Ottawa, and progressing in a southerly direction for approximately 6.5 km (4.0 mi). By mid-1923, the route was paved through Spencerville and North Gower, and work was underway to pave it within Manotick.

Until the summer of 1925, Ontario highways were named rather than numbered. When route numbering was introduced, the Prescott Highway became Provincial Highway 16. That year also saw paving completed through Kemptville to the Rideau River, as well as beyond North Gower in to Ottawa. This left unpaved segments south of Kemptville (except through Spencerville), and from the Rideau River to the village of North Gower. On October 22, 1928, the pavement between Johnstown and Spencerville was completed and opened to traffic. Premier Howard Ferguson officially opened the completed highway on October 7, 1929, at a rail overpass south of Kemptville. After cutting a ribbon spanning the bridge, he dubbed the route the Prince of Wales Highway.

In 1966 the Eastern Ontario Highway Planning Study was published by the Department of Highways (DHO), the predecessor to today's Ministry of Transportation of Ontario (MTO), identifying the need for a controlled-access highway between Ottawa and Highway 401. Highway 16, which crosses the geologically subdued St. Lawrence Lowlands, was selected over Highway 15, which crosses the undulating Canadian Shield to the west, as the ideal route for the new link. To overcome the issue of abutting properties established along the Highway 16 corridor, the DHO began purchasing a new right-of-way between Highway 401 and Century Road by late 1967 and constructed a two lane bypass of the original alignment, avoiding all the built up areas that the original Highway 16 encountered. This route, dubbed "Highway 16 New", was designed to easily accommodate the eventual upgrade to a freeway when traffic volumes necessitated.

Construction of the super two bypass took place between 1969 and 1983. The Spencerville Bypass opened by 1971, connecting with the old highway in the south near Crowder Road and in the north near Ventnor Road. By the end of 1973 the new highway was completed from immediately north of Highway 401 through Leeds and Grenville United Counties and into Ottawa–Carleton. This included a bypass around Kemptville and a new structure over the Rideau River. The new highway ended at Dilworth Road (Regional Road 13).

For nearly a decade, no new construction took place. Then, during the summer of 1982, the MTO awarded a contract to begin constructing the route north from Dilworth Road towards Manotick, bypassing North Gower. Following the completion of this first contract, which extended the route as far north as Roger Stevens Drive (Regional Road 6) and included a structure over Stevens Creek, a second contract was awarded for the remaining distance north to Century Road (Regional Road 8). The project was completed in 1983, merging into the original route of Highway 16 northeast of the present Prince of Wales Drive overpass.