Transport Minister Keith Brown announced the limit will increase from 40mph to 50mph when average speed cameras are introduced on a 136-mile stretch of the road from Dunblane to Inverness, possibly by the spring of 2014.
The trial will run alongside a safe driving campaign led by the haulage industry and A9 Safety Group.
The news comes as lorry drivers prepared to protest before Christmas on the road over the introduction of average speed cameras without a higher speed limit for HGVs. They had planned to stick to 40mph in "rolling roadblocks", to show what it will be like with the cameras, before the road is upgraded to dual carriageway.
Conor Mckenna, the Inverness-based organiser of the protest, said: "I am delighted for all concerned and all those who want to improve road safety on the A9 and my fellow HGV drivers who are aware that travelling at 40mph on the A9 was causing tailbacks and frustration.
"I thank the Transport Minister for eventually listening to us all," he added.
Phil Flanders, director of the Road Haulage Association in Scotland, also welcomed the news. He said: "We anticipate a smoother flow of traffic and a substantial reduction to the frustration among car drivers that the low speed limit creates."
Tory MSP Murdo Fraser, who heads the Dual the A9 campaign, said: "The deadly reputation of the A9 is built on its confusing layout and accidents being caused by frustrated drivers overtaking slow-moving goods vehicles. Hopefully some of that can be neutralised by this move."