Evans confirmed the news as he tweeted a picture of himself, LeBlanc and The Stig.
The revamped version of the BBC show is due to air in May.
LeBlanc said: “As a car nut and a massive fan of Top Gear, I’m honoured and excited to be a part of this iconic show’s new chapter. What a thrill!”
Evans said LeBlanc, who is best known for his role as goofy lothario Joey in Friends, is a “lifelong petrolhead”.
“Acting out our craziest car notions on screen is a dream job and I know we’ll both be debating some epic road trip ideas,” Evans said.
LeBlanc appeared on the original show in 2012 and set the fastest-ever time for their Star in a Reasonably Priced Car segment.
According to the official Top Gear website, he set an unbeaten lap time of 1m 42.1s in a Kia cee’d.
LeBlanc is no stranger to presenting for Top Gear and recently hosted a spin-off called Top Gear: The Races.
The seven-episode series saw the American actor going through the stand-out racing moments from the first 21 series of the show.
Evans was announced as the new face of Top Gear last year, following the departure of Jeremy Clarkson. Clarkson’s contract with the BBC was not renewed following a “fracas” with one of the show’s producers, Oisin Tymon.
Shortly afterwards his co-hosts, Richard Hammond and James May, confirmed they would not be returning to the show.
The trio have signed what is reportedly a lucrative deal and are set to launch their own motoring show on Amazon Prime later this year. May recently told the Radio Times he would like to see Evans’s reboot of the show doing well. “It’s a ballsy call to continue it. I wouldn’t want to be the one presenting it when we’d just finished, but there must be a way of reinventing it,” he said.
In the run-up to the announcement of Evans’ official co-hosts, other names speculated to be joining him were former F1 driver David Coulthard and motoring journalist Chris Harris.
Coulthard was recently confirmed as a presenter for Channel 4’s F1 coverage, officially ending speculation that he was set to join the Top Gear team alongside Evans.
Recent reports suggesting the new show may not be ready for its May debut were also dismissed by the BBC.
A BBC spokesman said: “Filming on Top Gear continues as planned and on schedule.”
Prior to it airing, the new show has also seen one of its executive producers, Lisa Clark, leave after just five months of working on the show.
Bookmakers William Hill said it was such a surprise bit of news, that not one bet had been made on LeBlanc.
William Hill spokesman Rupert Adams said: “This is an astonishing bit of secret keeping - nobody predicted Matt LeBlanc and if anyone had enquired we would have made him a 500/1 outsider.”
The bookmakers speculate that a female co-presenter will be announced next and have German professional driver Sabine Schmitz as their 1/3 favourite.