The A31 Magherafelt bypass was officially opened by Infrastructure Minister Chris Hazzard.
The four-mile route loops round the town, carrying traffic from the west towards the main roads for Derry and Belfast.
That traffic previously had to pass through Magherafelt, often with lengthy delays.
About 25,000 vehicles a day travel through the town centre.
Opening the bypass on Thursday morning, Mr Hazzard said it would ease congestion in the town by as many as 50,000 vehicles a week.
"In addition to reducing congestion, the bypass will help improve road safety and reduce journey times," he said.
That is expected to fall to around 18,000 with the opening of the bypass.
Many traders in the County Londonderry town have welcomed the new road, saying it will make Magherafelt a more attractive shopping option.
But some are concerned at the loss of passing trade.
Sara Jane Knox, a member of the town's Chamber of Commerce who runs a shop in the town centre, was looking forward to the road opening.
"It can get quite bad, mornings and afternoons especially," she said.
"To queue for half an hour to get into Magherafelt before you even think about parking - I'm sure it puts some people off at certain times of the day."
The new road links a roundabout on the Moneymore Road with an existing roundabout at Castledawson.
A £35m road scheme, which will take 7,000 cars a day out of a busy town, has opened weeks ahead of schedule.