Drivers are advised to allow more time for journeys
Motorists travelling in and out of Belfast through Stockman's Lane have been urged to take care as 27 months of construction work ends.
By replacing the bridges which carry the M1 over this junction three lanes of traffic can now be accommodated.
Traffic lights have been installed at the roundabout and will be monitored over the next few days to co-ordinate the flow of traffic.
Roads Minister Conor Murphy announced the opening of the new junction.
"The complete replacement of two bridges on a live motorway while keeping traffic running is an amazing achievement - I would again like to commend all involved with the project," he said.
Kevin Monaghan from the Roads Service said drivers should allow extra time for journeys over the next few days until they become familiar with the road layout.
"The big time for motorists will be the peak hour in the morning and the peak hour in the evening," he said.
The temporary traffic arrangements on the motorway of two lanes in each direction will remain unchanged as a result of the completion of this work.
The rest of the M1 and Westlink upgrade is scheduled to finish next year.