The Canadian province of Ontario has an extensive network of Primary (King's), Secondary, and Tertiary Highways, with county-level and city-level roads linking between them. Over the years, however, Ontario has turned back numerous highways to municipal government bodies, renumbered them, or upgraded them to 400-series highways.

In 1997 and 1998, many sections of the provincial highway network were downloaded to local municipalities (such as cities, counties or regional municipalities) by the Ontario Ministry of Transportation as a cost-saving measure. While highways were occasionally transferred to local governments in the past, the 1997–1998 downloads represented the most significant changes to Ontario's highway network. Many highways were completely devolved, while of others only short sections remain under provincial jurisdiction (Highway 2, once stretching across Southern Ontario, now is only a few kilometres long). Below is a partial list of partially or wholly devolved highways since 1997.

King's highways

– Windsor to the Ontario-Quebec boundary, now Lake Shore Boulevard, Kingston Road, Toronto Road, Dundas Street, and Loyalist Road. Its purpose was largely superseded by Highway 401 and 403; only a 1.1-kilometre section near Gananoque remains.

Scarborough, Ontario – Was once an expressway in the 1940s until Highway 401 took over most of its current route in 1952. Windsor – Tilbury – Designated in 1929, renumbered to Highway 98 in 1938. London, Ontario – Paralleled Highway 2 through the downtown core of London. It was established in 1956 and decommissioned in 1968. Cornwall, Ontario – Bypassed the Seaway International Bridge. It was established in 1965 and decommissioned on 1967. – Formerly Ontario Highway 2A in Chatham, Ontario from 1961 to 1970; decommissioned and route reverted to local street names (Richmond Street, Queen Street, School Street, 5th Street and Thames Street)

– between St. Thomas and Leamington and within the urban areas of Port Colborne and Fort Erie. Highway 3A A section of Ontario Highway 3 between Chambers Corners and Niagara Falls A designation of Ontario Highway 3B in Windsor

– Downgraded portions: south of Highway 3 in Talbotville Royal, and north/east of Highway 8

– Downgraded all of the Paris-Scarborough route except one 12.7 km section between Ontario Highway 8 and Ontario Highway 6

– Downgraded portions: south/west of Port Dover

– Downgraded the portion in York Region west of Donald Cousens Parkway / York Regional Road 48.

– Highway 7 west of Carleton Place to Highway 15 in Carleton Place. – From Winona to Niagara Falls, and, more recently, from Peters Corners to Winona.

– Split into two segments in Orangeville area and eastern end cut past Highway 400 towards Newmarket due to downgrading of various portions.

– Hurontario Street in Mississauga and Main Street in Brampton were formerly part of this route, but lost the designation with the completion of Highway 410 through the area. The Highway 10 designation is still in use north of Brampton.

– from Barrie southward; Now known as Yonge Street, York County Road 1, and Simcoe County Road 4. – Toronto; Gardiner Expressway/Lake Shore Boulevard to Highway 401. Now known only as York Street, University Avenue, Avenue Road, and Oriole Parkway. – Cobalt-New Liskeard. Portions of the road in Coleman Township are still signed as 11B. – Whitby—south of Highway 407 right-of-way.

– Marmora to Highway 62 near Foxboro, the 14 numbering originally continued south (on what is now Highway 62) to Picton. Only one of the *15 Ontario highways (2–17 with 13 skipped) to be completely dropped from the system. – From CFB Kingston to Highway 401, now Kingston Road 15, and from Carleton Place to Arnprior, now Lanark County Road 29 and Ottawa Road 29.

– Highway 15 west of Carleton Place to Highway 29 in Carleton Place. Renumbered as Highway 7B in 1961. – Upgraded to a 400 series highway (Highway 416). Only exists as a short connector highway between Highway 416 north of Highway 401 and Leeds & Grenville County Road 2 (formerly Highway 2). The section north of North Gower, which is different from the original route, is designated Ottawa Road 73 (Prince of Wales Drive). – Replaced by Highway 417 south and east of Arnprior to the Ontario/Quebec border.

– From Windsor to Leamington, now Essex County Road 20

– From terminus at Port Burwell to Highway 3 in Tillsonburg; from Highway 401 to terminus at Highway 86 near Milverton. The section from Tillsonburg to Highway 401 remains. – From QEW exit 88 east of Hamilton to Highway 58 near Allanburg and from Thorold Townline Road to Niagara Falls, now Regional Road 20 (Confederation Parkway through Stoney Creek, Rymal Road, and Lundy's Lane); only a 1.9-kilometre section near Allanburg remains.

– Portions South of Highway 402

– London, Ontario to near Warwick, Ontario, connecting to Sarnia, Ontario and to Arkona by Highway 7; replaced by most of routing of Highway 402

– Portion between Highway 89 in Palmerston to the Highway 9 Junction at Teviotdale downloaded; rerouted over section of Highway 89 in 2003 – 2 km south of Cambridge and northward through Guelph, Orangeville, Shelburne, and Collingwood on Georgian Bay.

– Now Bronte Road (south of Ontario Highway 407 in Oakville), Ontario St/Steeles Ave/Martin St/Halton Regional Rd 25 in Milton, Halton Regional Rd 25 in Halton Hills, and Main Street in Acton. Continues in segments through Wellington & Dufferin Counties to north of Grand Valley at Highway 89. – Etobicoke to Midhurst; now referred to as Highway 27 in Toronto, York Regional Road 27 in Vaughan, and Simcoe County Road 27 in Simcoe County – Port Hope to Highway 7A, (which had a concurrency with Highway 115 for 13  km to the Parkway exit), up to NE of Lakefield, at Highway 134.

– Brockville to Arnprior. The section from Smiths Falls to Carleton Place is now Highway 15; the rest was demoted as County Road 29. – Brighton to Havelock – Morrisburg to Ottawa – Gananoque to Highway 15, near Seeleys Bay; now referred to as Leeds and Grenville County Road 32 – Trenton to Picton; the portion east of Collins Bay Road in Kingston, Ontario was also removed from the provincial system – Lancaster to Hawkesbury – Portion between Highway 401 and Highway 115 interchange in the Regional Municipality of Durham. Was concurrent with and is now part of Highway 115.

– Lindsay to Highway 28 at Burleigh Falls. Now referred to as Kawartha Lakes Road 36.

– Kingston to Highway 7 near Sharbot Lake; now referred to as Kingston, South Frontenac and Central Frontenac Road 38 – Windsor to south of Belle River. (1934–1970. Now Essex County Road 22 from Essex County Road 42 to Essex County Road 2, and Essex County Road 2 into Windsor. Rendered obsolete with the completion of E.C. Row Expressway in 1970, and was absorbed into Highway 2.

– Portion between Highway 2 and Highway 7 downgraded as Lennox and Addington County Road 41.

– Entirely downgraded, Westport to Forthton. – Entirely downgraded. Perth to Alexandria, via Smiths Falls, and Winchester deleted in 1998—Perth to Smith Falls section had been part of Ontario Highway 15 prior to renumbering of the Perth-Carleton Place-Kanata portion of 15's route as Highway 7 in 1961.

– Between Lanark County Road 29 (formerly ) in Almonte, Ontario and Donald B. Munro Drive (formerly ) west of Carp, Ontario (1938–1997). Now Ottawa Road 49 (March Road). – Between Northumberland County Road 2 (formerly ) in Cobourg and in Norwood. Now known as Northumberland County Road 45 and Peterborough County Road 45. – Between and Kawartha Lakes Road 48 (formerly ) near Bolsover. Now Kawartha Lakes Road 46. – Highway 48 north of Stouffville to Highways 7 & 12 south of Greenbank. – Between Kingston Road and York Regional Road 25.

– Between Picton and the Quinte Skyway Bridge. – Toronto section; also referred to as Albion Road to Highway 89 west of Alliston. – Ran from Highway 3 in Eatonville to Rondeau Provincial Park Entrance. – East of Alberton to Highway 8 at Peters Corners.

– Highway 20 in Stoney Creek to Eastwood. – Cayuga to Cainsville. – Now Niagara Regional Road 55. – Highway 3 west of Canfield to Stoney Creek. – Bismarck to Becketts Bridge. Deleted in 1970. – From Highway 58A in Welland to Highway 406 between Turner's Corners and Allanburg. – Transferred in its entirety to the counties of Norfolk, Oxford and Perth.

– became Highway 6 on Manitoulin Island. – replaced by Highway 400 south of MacTier. Highway's length continues to shrink with extension of Highway 400 to Sudbury, and the number will disappear entirely in the future.

– renamed Highway 6 from Hepworth to Springmount.

– Port Bruce to Dorchester – New Sarum to Nilestown. Deleted in 1997. – Proposed in 1970 from Canborough to Bismarck (never built). – Highway 3 in Eagle to Highway 2 northeast of Wardsville. – Wallaceburg to Dresden. – Highway 2 south of Bothwell to Port Franks.

– Courtright to Strathburn.

– Highway 2 west of Delaware to Grand Bend. – Highway 7 south of Thedford to Port Franks. Became part of Highway 79 in 1982. – Ontario Highway 21 near Grand Bend to Ontario Highway 23 in Russeldale. – St. Joseph to Hensall. – Became part of Highway 86 in 1981; Waterloo to Elmira was downloaded in 1998, remaining section of Highway 86 renumbered back to Highway 85. – Original section from Elmira to Amberley downloaded in 1998; rest downloaded or became Highway 85.

– Bluevale to Harriston. – Ontario Highway 27 junction at Bond Head to Highway 11 junction in Bradford. Existed from 1938 to 1998. Renamed as Simcoe County Road 88. – Eastern end truncated at Ontario Highway 400 (original terminus was Ontario Highway 11) – Barrie to Angus and CFB Borden. – Duntroon to Stayner. – Wasaga Beach to Elmvale – South of Highway 400 interchange – Wolfe Island – Wolfe Island – Freelton to Hickson, through Galt/Cambridge. – Essex County 1938–1970. It was deleted, becoming Essex County Road 46, and Kent County (now Chatham-Kent) Road 8. Its western terminus was downtown Windsor, and its eastern terminus was downtown Blenheim, at Highways 40 and 3. Rendered obsolete with the completion of nearby freeway, Highway 401 (which paralleled Ontario Highway 98 for the majority of its length).

– Governor's Road (original alignment of Dundas Street from Highway 24 North of Brantford to Ancaster – Ran from MCF (ON-401) at exit 194 to Former ON-2, eastern part of London, now Veterans Memorial Parkway. – Highway 400 and 12 to Highway 69 junction, first the original alignment of Highway 69, now Highway 400. – to Grand Valley from Ontario Highway 9, then Ontario Highway 25 . – Welcome to Highway 28 north of Port Hope. Now Northumberland Road 74. – Highway 18 in Ruthven, Ontario, to Highway 3 in Ruthven. Now Essex County Road 45. – Hwy 500 east of Tory Hill to Paudash. Became part of Highway 121 in 1964. – Highway 15 west of Carleton Place to Highway 29 in Carleton Place. Renumbered as Highway 15B in 1960. – Hwy 500 south of Highland Grove to Dyno Mines. Renumbered Secondary Highway 648 in 1964. – Highway 3 (Former alignment, now Essex County Road 34) in Maidstone, Ontario (now a part of Tecumseh, Ontario, to Highway 98 (Now Essex County Road 46). Part of road is still signed as "Old HWY 114" from Manning Road (Essex County Road 19) to Provincial Road (Essex County Road 46). – Hudson to Highway 72 in Patricia. – Highway 11 north of Bracebridge to Dorset. – Western end truncated at Ontario Highway 11 (original terminus was Ontario Highway 169) – Highway 17 west of Dryden to Richan. Renumbered Secondary Highway 665 in 1974. – Highway 622 in Atikokan to Highway 17 Shabaqua Corners. Renumbered as part of Highway 11 in 1960. – Highway 35 south of Fenelon Falls to Haliburton. Eastward is renamed Highway 118. – QEW in Oakville to QEW in Mississauga. Decommissioned in 1970. – Now Airport Road in North Bay, Ontario.