The North Carolina Department of Transportation notified the town that the Glasgow Road bridge in MacGregor should be closed to all traffic.
Detours have been set up around the bridge, which will be closed until further notice.
Town officials say it will be a minimum of several months until repairs can be made.
The DOT conducts bridge inspections every two years. During the inspection period, all bridges are reviewed, including about three dozen in Cary.
Problems with the MacGregor bridge were identified during the routine inspection.
Inspectors found what they call "substantial and unexpected deterioration" since the previous inspection two years ago.