The 29-year-old has been part of the Sky line-up since its inception in 2010 and has seen his role progress from domestique to Classics challenger to potential Grand Tour winner in six years.
Thomas has seen his stage-racing credentials boosted in recent years with wins in the Volta ao Algarve in 2015 and 2016 and a nail-biting win at Paris-Nice in March.
“Signing again was an easy decision. Team Sky feels like home. From the other riders to the staff, I love it here,” Thomas said of his extension.
“I’ve only really just started when it comes to trying to win stage races. I’m learning all the time about how to get in the best shape for them and how to lead a team.
“For me, this year started with winning Paris-Nice and is now about going to the Tour [de France] in the best shape possible to support Froomey [Chris Froome]. After the Tour we can start looking to next year and keep setting bigger and better goals.
“It’s about keeping that progression going, and Team Sky is the best place to do that. I’m just super happy being part of the team.”
Show us your Scars: Geraint Thomas
Team principal, Sir Dave Brailsford, says that Thomas epitomises what Team Sky is about and praised the Welshman’s versatility as a rider.
“Geraint is one of the most versatile bike racers in the world. He has demonstrated this by winning on the track, in the Classics and now in stage races.
“He has gone from strength to strength and winning Paris-Nice this year was another step up. I think it gave him real confidence and now he can set his sights on developing his performances in the Grand Tours.
“I still believe the best is yet to come from Geraint and we will enjoy supporting him to see just how far he can go.”