The bridge, over the railway line at Finaghy Road North, will be closed to traffic from Monday 23 June.
The work will involve replacing existing walls and fences over the bridge with high standard safety barriers.
The Roads Service has said, due to the restricted width of the bridge, this work cannot be carried out safely when it is open to traffic.
The Road Service has timed the closure to coincide with the summer period, when traffic levels are usually at their lowest.
The road may not be reopened until September, due to the complex nature of the work and, what the roads department described as "construction constraints".
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The Roads Service has said it will work closely with the contractor to open the road as early as possible.
Diversionary routes via Blacks Road and Stockmans Lane will be signposted and traffic alerted in advance by roadside signs.
The junctions on these alternative routes will be closely monitored to ensure that diverted traffic does not suffer undue delay, according to the department.
The Road Service has asked motorists to be patient and to allow sufficient additional time for their journey.
Existing train services will operate as normal and Finaghy halt will remain operational throughout the works. A pedestrian route will be retained adjacent to the bridge with a temporary footbridge spanning the railway line, and a free shuttle bus will operate from 8am - 8.30pm daily between Orchardville Avenue and Diamond Gardens to cater for those with mobility difficulties.
The temporary footbridge and shuttle bus will come into operation on 23 June.
(PR/JM)
