For roads named A21 see ; List of A21 roads
Route 21, or Highway 21, can refer to:

• E21 European route E21

• Chandler Highway
• Toowoomba Connection Road (QLD)
• – Kakadu Highway

• Wiener Außenring Autobahn

• National Road 21 (Cambodia)

• Alberta Highway 21
• British Columbia Highway 21
• Manitoba Highway 21
• Newfoundland and Labrador Route 21
• Ontario Highway 21
• Prince Edward Island Route 21
• Saskatchewan Highway 21

• National Route 21

• I/21 Highway; Czech: Silnice I/21

• EO21 road

• Main road 21 (Hungary)

• National Highway 21 (India)

• Road 21

• N21 road (Ireland)

• Autostrada A21

• Japan National Route 21

• National Route 21

• M21 highway (Moldova)

• New Zealand State Highway 21

• National Route 21

• O-21, a motorway in Turkey running from Kemerhisar, Niğde Province to Tarsus, Mersin Province.

• British A21 (Hastings-London)
• A21 road (Northern Ireland)

• U.S. Route 21
• Alabama State Route 21
• Arkansas Highway 21
  • Arkansas Highway 21E (former)
  • Arkansas Highway 21W (former)
• California State Route 21 (former)
  • County Route A21 (California)
  • County Route E21 (California)
  • County Route G21 (California)
  • County Route J21 (California)
  • County Route S21 (California)
• Colorado State Highway 21
• Connecticut Route 21
• Florida State Road 21
  • County Road 21B (Clay County, Florida)
  • County Road 21B (Duval County, Florida)
  • County Road 21D (Duval County, Florida)
  • County Road 21 (Marion County, Florida)
  • County Road 21 (Putnam County, Florida)
• Georgia State Route 21
• Hawaii Route 21 (former)
• Idaho State Highway 21
• Illinois Route 21
• Indiana State Road 21 (former)
• Iowa Highway 21
• Kentucky Route 21
• Louisiana Highway 21
• Maryland Route 21
• Massachusetts Route 21
• M-21 (Michigan highway)
• Minnesota State Highway 21
  • County Road 21 (Anoka County, Minnesota)
  • County Road 21 (Hennepin County, Minnesota)
  • County Road 21 (Scott County, Minnesota)
• Mississippi Highway 21
• Missouri Route 21
• Montana Highway 21
• Nebraska Highway 21
  • Nebraska Spur 21B
  • Nebraska Spur 21C
• Nevada State Route 21 (former)
• New Jersey Route 21
  • County Route C21 (Bergen County, New Jersey)
  • County Route 21 (Monmouth County, New Jersey)
• New Mexico State Road 21
• New York State Route 21
  • County Route 21 (Allegany County, New York)
  • County Route 21 (Chautauqua County, New York)
  • County Route 21 (Columbia County, New York)
    • County Route 21B (Columbia County, New York)
    • County Route 21C (Columbia County, New York)
  • County Route 21 (Delaware County, New York)
  • County Route 21 (Dutchess County, New York)
  • County Route 21 (Erie County, New York)
  • County Route 21 (Essex County, New York)
  • County Route 21 (Genesee County, New York)
  • County Route 21 (Hamilton County, New York)
  • County Route 21 (Herkimer County, New York)
  • County Route 21 (Lewis County, New York)
  • County Route 21 (Oneida County, New York)
  • County Route 21 (Ontario County, New York)
  • County Route 21 (Orange County, New York)
  • County Route 21 (Oswego County, New York)
  • County Route 21 (Otsego County, New York)
  • County Route 21 (Putnam County, New York)
  • County Route 21 (Rensselaer County, New York)
  • County Route 21 (Saratoga County, New York)
  • County Route 21 (Schoharie County, New York)
  • County Route 21 (Suffolk County, New York)
  • County Route 21 (Sullivan County, New York)
  • County Route 21 (Ulster County, New York)
  • County Route 21 (Washington County, New York)
  • County Route 21 (Westchester County, New York)
  • County Route 21 (Wyoming County, New York)
  • County Route 21 (Yates County, New York)
• North Carolina Highway 21 (former)
• North Dakota Highway 21
• Ohio State Route 21
• Pennsylvania Route 21
• South Carolina Highway 21 (1920s) (former)
• South Dakota Highway 21
• Tennessee State Route 21
• Texas State Highway 21
  • Texas State Highway Loop 21
  • Farm to Market Road 21
  • Texas Park Road 21
• Utah State Route 21
• State Route 21 (Virginia 1918-1933) (former)
• Washington State Route 21
  • Primary State Highway 21 (Washington) (former)
  • Secondary State Highway 21B (Washington) (former)
  • Secondary State Highway 21C (Washington) (former)
• County Route 21 (Jackson County, West Virginia)
  • County Route 21 (Kanawha County, West Virginia)
  • County Route 21 (Wirt County, West Virginia)
  • County Route 21 (Wood County, West Virginia)
• Wisconsin Highway 21

Territories

• Puerto Rico Highway 21

• List of A21 roads
• List of highways numbered 21A