The inaugural Severn Bridge Half Marathon will cross the bridge on the M48 main carriageway, and to allow this the bridge will be closed between 8.20am and 10.50am.
This is the first running race to take place on the bridge since it opened in 1966.
The 13.1 mile route starts in Chepstow on the M48 motorway, crosses to England over the River Severn, then takes in roads and picturesque country lanes near Thornbury, and finishes back in Wales after another Severn crossing via the cycle and footpath on the bridge.
While the bridge is closed the following signed diversions will be in place:
Road users travelling westbound to Chepstow should use the M4 Second Severn Crossing, exit at J23a (Magor), then follow the M4 eastbound, then follow the M48 to Chepstow.
Eastbound traffic should follow the M48 westbound then join the M4 eastbound at J23a (Magor) and use the M4 Second Severn Crossing.
Road users are advised to plan their journeys.
Members of the public should contact the Highways Agency Information Line on 0300 123 5000.
Journalists should contact the Highways Agency press office on 0844 693 1448 and use the menu to speak to the most appropriate press officer.