The 9.7km bypass will be built from the Goat roundabout on the A499/A487 junction to the Plas Menai roundabout, around Llanwnda, Dinas, Bontnewydd and Caernarfon avoiding the town centres.
The route will be made up of three sections separated by new roundabouts at Meifod and Cibyn.
The government said a Design and Construct contract will be awarded with detailed scheme design commencing in June.
Construction could then start in November 2018, and be completed by spring 2021.
Construction information specialist Glenigan has Balfour Beatty and Jones Brothers listed as main contractors.
First Minister Carwyn Jones said: “It’s great news we will be pressing ahead with the Caernarfon and Bontnewydd bypass.
“The bypass will not only help connect communities in the area through sustainable and resilient infrastructure, but also provide a vital link to the A55 and beyond to Ireland, England and Europe.
“It also presents a big opportunity for local employment, workforce training and apprenticeships, especially during the construction phase.”