Engineers say sonar scanning shows scouring has washed away too much sand from the supports for the bridge to be safe for vehicles to cross over.
The DOT had reported the problem several days ago, but said it was not severe enough to close the bridge. It said Tuesday that additional scanning showed the problem was worse than first thought.
The NCDOT says it's declared a state of emergency to speed up repairs.
"Closing the Bonner Bridge is necessary to keep all travelers safe, but we know it will have a devastating effect on the people who live along and visit the Outer Banks," said NCDOT Secretary Tony Tata.
Bonner Bridge is the only highway access for vehicles between Hatteras Island and the mainland. Until it is safe to reopen, The NCDOT Ferry Division will provide emergency support to move people and cars across the Pamlico Sound.