Route 21A, or Highway 21A, may refer to:

• National Highway 21A (India)

• County Road 21A (Clay County, Florida)
• M-21A (Michigan highway) (former)
• Nebraska Spur 21A
• New Jersey Route 21A (former)
• New York State Route 21A (former)
  • County Route 21A (Suffolk County, New York)
  • County Route 21A (Sullivan County, New York)
  • County Route 21A (Warren County, New York)
• Secondary State Highway 21A (Washington) (former)

• List of A21 roads