Modern technology has been installed on a 13.4-mile section of the motorway between the M25 and Farnborough, while the hard shoulder has been converted to a new fourth traffic lane in each direction.
The scheme got underway in November 2014, with main construction works completed overnight on 30 June.
Shaun Pidcock, Director of Highways England's smart motorway programme said: "This is a significant upgrade for the M3 and will deliver real benefits for the 130,000 drivers who use it every day. We've added 26.8 miles of new lanes to this busy section of the M3 and completely overhauled the technology to give drivers better information to help with their journeys.
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"Smart motorways create vital extra capacity, improve journey times and maintain our high levels of safety across the network. New technology will make the road more resilient to disruption, improving journeys by using variable speed limits that will help tackle frustrating stop-start traffic and giving drivers better information on conditions ahead.
"As well as the smart motorway upgrade, we have also been carrying out the most extensive maintenance on the M3 since it was first built in 1971, fully resurfacing the motorway and its slip roads and restoring it to an 'as new' condition.
"I would like to thank drivers for their co-operation and understanding while we've been building this new improved road and hope they enjoy the improved journeys between London and the south coast."
(LM/MH)
