Secondary Highway 503, commonly referred to as Highway 503, was a provincially maintained secondary highway in the Canadian province of Ontario. The 116.8 km (72.6 mi) route existed between 1956 and 1998. Between 1956 and 1963, the highway stretched from Kirkfield to Sebright, and then along the Monck Road from Sebright to Kinmount, entirely within Victoria County. In 1964, the route was extended to Highway 121 in Tory Hill along the route of Highway 500 through the counties of Peterborough and Haliburton. In 1998, the route was transferred to the various counties in which it resided. Today it is known as Kawartha Lakes City Road 6 and 45, Peterborough County Road 503 and Haliburton County Road 503.