Work is now underway on the five-mile long section of carriageway between Kincraig and Dalraddy.
This is expected to be complete by summer 2017.
The A9 Dualling programme will see approximately 80 miles of road upgraded between Perth and Inverness by 2025.
The £35m Kincraig-Dalraddy contract involves upgrading a stretch of single carriageway to dual carriageway and is being carried out by a joint venture (JV) between Wills Bros Civil Engineering and John Paul Construction.
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Delivering the project requires over 700,000 tonnes of excavation and will see the contractor lay over 18 hectares of new road carriageway.
Announcing the start of works, Mr Brown said: "With the official opening of the Borders Railway and now the ground-breaking of work on the A9 this has been a historic week for transport infrastructure in Scotland.
"It is no understatement to say that we are marking an important milestone in what will be one of the largest and most challenging infrastructure projects in Scotland's history."
He added: "With nearly three quarters of a million tonnes of excavation work to be carried out on this section here at Kincraig alone, we are at the start of a long journey to deliver the entire programme."
A spokesperson for the Wills Bros / John Paul JV said: "We are delighted to see construction get underway on the A9 Kincraig to Dalraddy project and proud to have our Joint Venture at the forefront of this prestigious dualling programme."
(LM/MH)
