The move comes as a review continues into the rest of the £3 billion project to complete some 70 remaining miles of dualling of the road between Inverness and Perth.
It is understood to have escaped the axe under last week’s SNP-Scottish Greens’ deal, but ministers have repeatedly failed to confirm the project is still on track to be finished on time by 2025, suggesting completion will be delayed.
The latest section to be opened, between Luncarty, at the northern end of the current dual carriageway from Perth, and the Pass of Birnam, has cost £96 million and taken two and a half years to build.
It comes four years after the previous section was opened, over nearly five miles between Kincraig and Dalraddy, south of Aviemore.
Mr Dey said the current lowered speed limits and other restrictions through the construction site would be removed in phases on Saturday and Sunday, with the new stretch open by Monday.
The work also included 2.5 miles of new and upgraded paths for walkers, horse riders and cyclists linking Luncarty and Bankfoot for the first time.
Other paths have also been upgraded with new asphalt surfacing, with mounting blocks have been installed at several locations to improve access for riders.
Mr Dey said: “I’m thrilled we’ve reached this most important milestone on the Luncarty to Pass of Birnam project.
"When the new section of road opens fully, it will bring tremendous benefits - improving connectivity between the Highlands and Islands and Scotland’s Central Belt, delivering far-reaching benefits to the wider Scottish economy.
“More locally, 4km [2.5 miles] of new and upgraded non-motorised user route have been constructed to connect the surrounding communities to Scotland’s core path network.
"Promoting active travel and supporting our commitment to build an active nation where people choose to walk, wheel and cycle for everyday journeys.
“The opening of this major infrastructure project will improve road safety, journey times and journey reliability for the millions of road users that use this strategic route north each year.
“In addition, this vastly improved route will help stimulate the economic recovery of the north of Scotland following the pandemic, mitigate the impacts of Brexit, support businesses, communities and tourism throughout Scotland by improving access to and from the Highlands and Islands.”
The contract, built by Balfour Beatty, included four new bridges over the A9 and the removal of all direct junction accesses to the A9 carriageway.
The Scottish Government’s Transport Scotland agency, which is in charge of the scheme, said that would improve road safety.
It said dualling the stretch would reduce driver frustration and stress by providing more opportunity to overtake safely.
The agency said Balfour Beatty would continue to complete landscaping over the next few months, which may require some traffic restrictions to enable the work to be done safely.
Such finishing work is expected to be completed over the winter.