King's Highway 43, also known as Highway 43, was a provincially maintained highway in the Canadian province of Ontario. On January 1, 1998, the entire route was transferred to the county that each section resided in, resulting in the current designations of Lanark County Road 43, Leeds and Grenville Road 43 and Stormont, Dundas and Glengarry Road 43. Highway 43 ran somewhat parallel to and between Highway 401 and Highway 417 from Highway 7 in Perth to Highway 34 in Alexandria, passing through several small towns along the way. At 154.2 km (95.8 mi),it is the longest highway in Ontario to be decommissioned entirely during the mass transfer of Highways in 1997 and 1998.