The pair last rode the British stage race together in 2009, since then winning seven grand tours between them.
Three time Tour of Britain stage winner Wout Poels will also be lining up for Team Sky when the race begins in Pembrey Country Park, Carmarthenshire, on Sunday 2 September.
Poels won his first Tour of Britain stage aged 23, in a solo breakaway during 2010's stage four.
Announcing the line up, Poels captured a video of himself riding with Thomas and Froome, saying: "Exiting news, I'm going to start in the Tour of Britain. But, I also need some good support..."
Thomas confirmed in the video that he'd ride as support, and Froome said he'd ride as back-up leader.
The rest of the Team Sky line-up remains to be seen.
Thomas - who was welcomed back to Wales following his Tour victory by streets lined with around 8000 fans - has ridden the Tour of Britain eight times since riding for the Great British national team in 2005.
He said: "As soon as I’d finished the Tour I knew I wanted to ride the Tour of Britain and race on home roads. It starts in Wales which will be special, and then I get to go and race across the whole of the UK. I can’t wait.
"I want to go to the race in the shape to compete and enjoy it. We’ll have to see how the next few weeks go but I’m looking forward to it and I know we will have a strong team there. Wout is looking really good at the moment too."
Thomas finished sixth overall in 2009 and seventh in the 2017 race. Froome has raced the event twice - in 2007 and 2009 - finishing 31st and 50th respectively - but he's won six grand tours since that time.
"It’s been a long time since I’ve raced the Tour of Britain," said Froome. "The Vuelta a Espana has always been such a big goal and sadly coincided with the Tour of Britain, but not doing La Vuelta this year gives me the chance to come back to the UK and race on what looks like a great parcours.
"I’m really looking forward to riding. I always remember there being a great atmosphere at the Tour of Britain and the race has only got bigger over the years. I’m really looking forward to coming back."
Race Director Mick Bennett said: "We’re honoured and thrilled that Geraint Thomas and Chris Froome will race for Team Sky at this year’s OVO Energy Tour of Britain.
"To have two of Britain’s greatest-ever riders - the reigning Tour de France and Giro d’Italia winners no less - competing in the race is fantastic news for our fans, who will line the streets in order to catch a glimpse of their heroes.
"We saw the amazing response that Geraint received during his homecoming celebrations in Cardiff last week - the fact that Stage One on Sunday 2 September takes place entirely within Wales means this time the whole country will be able to celebrate his incredible success in France from the summer!
"Since he last rode in the race, Chris has become - without any doubt - the leading Grand Tour rider of his generation. We're delighted to welcome him back to the OVO Energy Tour of Britain after his Tour, Giro and Vuelta successes in the past year.
"I’m also looking forward to watching Wout's return to the race, having seen his three previous OVO Energy Tour of Britain stage victories up close. He’ll be one to watch during this year’s hill-top finish at Whinlatter on Stage Six, that’s for sure!"
The provisional start list will be published on Friday August 24 - but we'll keep revealing names as Team's announce them.
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