Working alongside principal contractor AmcoGiffen, Network Rail engineers have set up a compound at the foot of the landmark, which sits 100ft above the River Finnan on the West Highland Line, 17 miles from Fort William.
The £3.4 million works is part of Network Rail's ongoing maintenance programme and will help futureproof the viaduct.
The project is expected to take just under 12 months to complete and will see engineers assessing the condition and strength of the viaduct and making improvements beneath the track to keep the stone ballast in place.
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Laura Craig, scheme project manager, Network Rail, said: "The main works to the viaduct will be concrete repairs. We'll also be doing some repairs to the pier that sits in the River Finnan.
"Years of planning have gone into this project. The work will not have a massive visual impact on the way the viaduct looks and that's very deliberate, with it being grade A listed.
"We won't be scaffolding the structure at all but in the coming weeks, you'll start to see the rope access technicians on the viaduct as they begin work."
There will be no disruption to rail services during the work on the structure.
Ahead of work commencing, a community drop-in event was held in Glenfinnan, allowing those who live near the viaduct the opportunity to ask questions directly to the project team.
Laura added: "We want to thank all who came to the drop-in event. It was great to catch up with locals to answer questions and chat about our plans.
"We know it's a much-loved viaduct and is, of course, the longest concrete viaduct in Scotland, spanning 1000ft.
"It's a bucket list job for our team to work on and we'll be sharing as much information and as many updates as we can throughout the project."
