The River Ceiriog Viaduct, north of Oswestry, was built 30 years ago and engineers say the large expansion joint on the Shropshire side is reaching the end of its life.
Traffic will be diverted through Chirk during the closure which will begin on February 22.
Built in 1990, the viaduct spans 500 metres from the northern side of the Ceiriog Valley in Wales to its southern side in England. After 30 years, the large expansion joint at the southern end of the viaduct is reaching the end of its operational life and must be replaced.
The work will include the demolition and reconstruction of the reinforced concrete viaduct where it joins its support on the southern side of the Ceiriog Valley.
Planned routine maintenance and bridge investigation works will also take place to make the most of the closure.
The Welsh Government and Highways England will carry out work on their own sections of the road.
Officials say the A5/A483 will be closed between the Halton and Gledrid roundabouts for the duration of the work, with traffic diverted through the town of Chirk along the B5070.
Temporary traffic lights will be installed on the B5070 Chirk Road bridge over the River Ceiriog.
"The bridge must be restricted to single-lane working for the duration of the works to ensure that the bridge can carry heavier vehicles without being over-loaded," Welsh road engineers say.
Transport Minister Ken Skates said: “It is essential that this work is carried out to maintain the condition of the A5 River Ceiriog Viaduct. Every effort is being made to minimise disruption as much as possible and to complete the work as quickly as possible.
“Planned full closures of carriageways are never taken unless they are absolutely necessary, but the safety of the travelling public is paramount and this work needs to take place to ensure that.
“I would urge the travelling public to follow the diversion route signage while this work is in progress, and I thank them and local residents for their co-operation and patience.”