Transport Secretary Michael Matheson said a new road through Glen Croe had been selected from among 11 options to bypass the notorious stretch, which included routes over the Clyde and via Bute.
Five options will now be considered for the new Glen Croe route, some of which include tunnels up to 1.8 miles long.
Transport Scotland said preliminary estimates showed they could cost between £268 million and £860 million at current prices.
Mr Matheson said the Glen Croe option would be cheaper and quicker to build than the other ten which were considered.
No timescale has been announced but the tunnel options could take up to two years longer than the others.
Mr Matheson told MSPs last week that solving the A83 problem was an “emergency project” that must be progressed at “rapid speed”.
Campaigners want it completed by May 2024 – in just over three years’ time.
Mr Matheson said as an interim measure, an alternative route could be constructed using the current Old Military Road diversion route, which runs parallel to the A83 above it, and forestry tracks, with final plans within 18 months.
He said: “Identifying the preferred route corridor is a major step forward for this vital work and we are now pushing forward to look at five alternative options within that online corridor and starting the process shortly to appoint design consultants for this work.
"Following substantial public feedback, our future assessment work will place particular emphasis on the timescale to deliver these options.”
John Gurr, chairperson of The Rest and be Thankful Campaign said: “Whilst we welcome today’s announcement and the preferred option of Glen Croe, we are still no further forward in terms of timescales.
“As we move into day 230 of the RABT being closed or disrupted, we urge Mr Matheson to put deliverable dates against this project.
"Our request of a completion date of May 2024 has once again fallen on deaf ears and without any mention of any dates, it feels very much like a fluffy pre-election promise.”
Former Argyll and Bute Liberal Democrat MP and Holyrood election candidate Alan Reid said six months had been wasted by considering the other options.
He said: "It is really frustrating we are back to where we were over eight years ago.
"All the SNP have done in that time is produce a slightly different coloured map.”
A83 replacement options
Brown: Largely follows A83 with 0.8-mile debris flow shelter and 300-metre viaduct.
Yellow: New 1.3-mile road between A83 and Old Military Road including 1.1-mile viaduct from Croe Water to the Rest and Be Thankful car park.
Green: New 2.7-mile road on opposite side of glen from A83, crossing the glen and Croe Water over a 200m viaduct, with landslip debris flow shelters, fencing and viaducts likely to be required.
Purple: New 2 mile road on Glen Croe floor including 0.7-mile tunnel.
Pink: New 2.5-mile road, 1.8 miles of which would be in a tunnel under the Croe Water
Full details here.