Ontario Highway 17

King's Highway 17, more commonly known as Highway 17, is a provincially maintained highway and the primary route of the Trans-Canada Highway through the Canadian province of Ontario. It begins at the Manitoba boundary 50 km (31 mi) west of Kenora and ends south of Arnprior at the western terminus of Highway 417, 1,964.0 km (1,220.4 mi) to the east. This makes it Ontario's longest highway.

The highway once extended even farther to the Quebec boundary in East Hawkesbury with a peak length of about 2,180 km (1,350 mi). However, a section of Highway 17 "disappeared" when the Ottawa section of it was upgraded to the freeway Highway 417 in 1971. Highway 17 was not re-routed through Ottawa, nor did it share numbering with Highway 417 to rectify the discontinuity, even though Highway 417 formed a direct link between the western and eastern sections of Highway 17. However, from East Hawkesbury to Ottawa, Highway 17 retained the Trans-Canada Highway routing and signs until it met up again and merged with Highway 417 until 1997, when Highway 17 through Ottawa was downgraded. The Trans-Canada Highway designation now extends along all of Highway 417.