The Ontario Provincial Highway Network consists of all the roads in Ontario maintained by the Ministry of Transportation of Ontario (MTO), including those designated as part of the King's Highway, secondary highways, and tertiary roads. Components of the system—comprising 16,900 kilometres (10,500 mi) of roads and 2,880 bridges—range in scale from Highway 401, the busiest highway in North America, to unpaved forestry and mining access roads. The longest highway is nearly 2,000 kilometres (1,200 mi) long, while the shortest is less than a kilometre. Some roads are unsigned highways, lacking signage to indicate their maintenance by the MTO; these may be remnants of highways that are still under provincial control whose designations were decommissioned, roadway segments left over from realignment projects, or proposed highway corridors.

Predecessors to today's modern highways include the foot trails and portages used by indigenous peoples in the time before European settlement. Shortly after the creation of the Province of Upper Canada in 1791, the new government under John Graves Simcoe built overland military roads to supplement water-based transportation, including Yonge Street and Dundas Street. At the time, road construction was under the control of the township and county governments. Local township roads were financed and constructed through a statute labour system that required landowners to make improvements in lieu of taxes. Private companies constructed corduroy and later plank roads and charged tolls in the second half of the 19th century. The rising popularity of the bicycle led to the formation of the Ontario Good Roads Association, which advocated for the improvement of roads and recreation as the automobile rose to prominence.

By the early 20th century, the province had taken interest in road improvement and began funding it through counties. The increasing adoption of the automobile resulted in the formation of the Department of Public Highways of Ontario (DPHO) in 1916. The passing of the Canada Highways Act in 1919 resulted in the establishment of a provincial network of highways. The DPHO assigned internal highway numbers to roads in the system, and in 1925, the numbers were signposted along the roads and marked on maps. In 1930, provincial highways were renamed King's Highways and the familiar crown route markers created. The DPHO was also renamed the Department of Highways (DHO).

The 1930s saw several major depression relief projects built by manual labour, including the first inter-city divided highway in North America along the Middle Road, which would become the Queen Elizabeth Way in 1939. In 1937, the DHO merged with the Department of Northern Development, extending the highway network into the Canadian Shield and Northern Ontario. Significant traffic engineering and surveying through the war years, during which construction came to a near standstill, led to the planning and initial construction of controlled-access highways. The 400-series highways were built beginning in the late 1940s and numbered in 1952.

The vast majority of modern road infrastructure in Ontario was built throughout the 1950s, 1960s, and early 1970s. The cancellation of the Spadina Expressway and the introduction of the Environmental Assessment Act in the 1970s resulted in a decline in new highway construction in the decades since. In the late 1990s, nearly 5,000 kilometres (3,100 mi) of provincial highways were transferred, or "downloaded" back to lower levels of government. Few new provincial highways have been built in the early years of the 21st century, although several major infrastructure projects including the Herb Gray Parkway and expansion of Highway 69 have proceeded. Recent construction has included the controversial Bradford Bypass and Highway 413.

In Ontario, all public roads are legally considered highways under the Highway Traffic Act (HTA), which sets forth regulations for traffic, or the rules of the road. The Public Transportation and Highway Improvement Act (PTHIA) sets forth the regulations concerning roads that are under the authority of the Province of Ontario. The act distinguishes and sets out the applicability of the HTA to provincial highways, which are designated as part of The King's Highway (primary), a secondary highway, or a tertiary road. Tertiary roads may also be designated as a resource road, allowing for vehicles otherwise prohibited from public roads. Industrial roads are privately owned routes with which the MTO has entered an agreement to improve or allow public access, and are not considered part of the provincial highway network. The 407 ETR is likewise not considered part of the provincial highway network. While it is still subject to the rules set forth by the HTA, it is otherwise governed independently under the legislation of the Highway 407 Act.

Speed limits on provincial highways are legislated by sections of the road, and vary between 50 km/h (30 mph) and 110 km/h (70 mph). Freeways, including the 400-series highways, are generally signed at 100 km/h (60 mph), although sections exist that are signed lower. Three segments of freeway are part of an ongoing pilot project to test speed limits of 110 km/h in rural areas that are not subject to congestion. A fourth segment, located in Northern Ontario, was set to be announced in 2020, but has been delayed since.

Ontario uses two distinct shapes of signage to mark the King's Highways. Confirmation markers, or reassurance markers, are utilized along the designated road to confirm (near intersections) or reassure (elsewhere) drivers that they are on the correct route. The markers, known as shields, feature the route number within an outline in the shape of a shield, topped by a St Edward's Crown. In other cases, particularly when approaching the junction of another highway, a square crown marker is used, featuring the route number within an outline of the St Edward's Crown, paired with an arrow plate; exit signs on freeways and at major junctions also use this crown symbol. For secondary highways, the route number is within an outline of an isosceles trapezoid, while tertiary roads place the number within an outline of a rectangle. When these markers appear along or at an intersection with the indicated highway, they feature black text on a reflective white background. There are two exceptions to this: The QEW, which features blue text on a yellow background; and the provincially maintained section of the tolled Highway 407, which feature white text on a blue background, with an orange plate with TOLL below in black. Signs prior to 1993 had the words "The King's Highway" below the crown, but current versions do not have the words.