Another lane will be added to the Heads of the Valleys road between Bryn Mawr and Tredegar.
It follows the announcement of almost £82 million of European funding.
The A465 dualling scheme will see improvements to the 7.8km route which extends from the north of the A465 roundabout at Brynmawr and tie in with the previously improved A465 at Tredegar.
The work is likely to start early next year.
More top news
Showers this evening with heavier and more persistent rain arriving overnight. A wet start tomorrow but improving during the afternoon.
A mother from Neath has told how her teenage son collapsed and was left unconscious for five hours after taking a so-called legal high.
Two brothers from Cardiff are campaigning for their cousin's murder to be re-investigated, 76 years after she died.