Transport minister Jenny Gilruth has approved statutory procedures for dualling the road on a 14-mile stretch between Killiecrankie and Glen Garry in Perthshire.
The route passes through the site of the Battle of Killiecrankie, a clash between Jacobite and government troops in 1689.
Campaigners have argued that changes proposed by Transport Scotland do not go far enough to protect the historic site.
The existing A9 lanes already pass through some of the battlefield and archaeological excavations have taken place in recent years.
A local public inquiry was held in January 2020.
Ms Gilruth said: “I am well aware of the sensitivities around what is an important battlefield at Killecrankie.
“The site is of historical and cultural significance, which is why Transport Scotland has been engaging extensively with the local community and key stakeholders since 2012.
“Having published draft orders for the Killiecrankie to Glen Garry scheme in November 2017, Transport Scotland made a number of design refinements to reduce land take on the battlefield site following feedback from Historic Environment Scotland and the local community.
“Following careful consideration of the findings, conclusions and recommendations, I have determined that the orders for the A9 scheme should be made.
“I want to thank everyone who submitted representations and to give an assurance that those representations have been carefully considered.
“Subject to there being no challenge, we will then be in a position to complete the statutory process for the scheme and acquire the land to build it.
“Work to determine the most suitable procurement options for the remaining sections of the A9 dualling is also ongoing.
“This is a complex exercise which looks at a number of factors, including how the project can be delivered most efficiently by the industry, whilst minimising disruption to road users and helping to support economic recovery post-Covid.”
Last week, opposition MSPs pressed the Scottish Government to recommit to fully dualling the A9 between Perth and Inverness during a debate at Holyrood.