The following highways are numbered 321:

• Nova Scotia Route 321
• Saskatchewan Highway 321

• China National Highway 321

• National Route 321

• Cuatro Caminos–Arroyo de Agua Road (1–321)
• Las Minas–Comunidad Pedro Pi Road (2–321)
• Santa Clara–Caibarién Road (4–321)

• Japan National Route 321

• U.S. Highway 321
• Arkansas Highway 321
  • Arkansas Highway 321 Spur
• Georgia State Route 321 (former)
• Kentucky Route 321
• Louisiana Highway 321
• Mississippi Highway 321
• Nevada State Route 321
• New Mexico State Road 321
• New York:
  • New York State Route 321
  • County Route 321 (Erie County, New York)
• North Carolina Highway 321 (former)
• Ohio State Route 321
• Pennsylvania Route 321
• Puerto Rico Highway 321
• Tennessee State Route 321
• Texas:
  • Texas State Highway 321
  • Texas State Highway Loop 321
  • Farm to Market Road 321
• Virginia State Route 321
  • Virginia State Route 321 (former)
• Wyoming Highway 321