The A6108 has been closed for emergency repairs near Half penny House.
The road is shut at Boston Farm.
Motorists can still get between Leyburn and Richmond and Catterick Garrison, but drivers wanting to go between Reeth and Catterick will need to go via Grinton Moor, or through Richmond or Tunstall.
North Yorkshire County Council says the closure is due to emergency repairs to a bridge.
No indication of how long the closure will be in place has yet been given.