The Welsh rider finished second at this year's Giro d'Italia and won the Tour de France in 2018. Thomas is the only British rider on the team for the Vuelta, and brings a wealth of experience to the race.
"Geraint has shown his form this season and proved the benefit of his many years of Grand Tour racing at the Giro d'Italia this year," Rod Ellingworth, the INEOS racing director, said.
"His experience, insights and knowledge, cool head and leadership skills are going to come into their own in a race like this," Ellingworth added.
Thomas has only competed in the Vuelta once previously, and is hoping for a more enjoyable experience. He finished in 69th place when riding for Team Sky, the former name of INEOS Grenadiers.
"I'm really looking forward to going back to La Vuelta. Hopefully it'll be better than 2015-which was horrific - it wasn't the most enjoyable Grand Tour that I've ever done," Thomas said.
The team includes Thymen Arensman, Egan Bernal, Jonathan Castroviejo, Laurens De Plus, Omar Fraile, Filippo Ganna and Kim Heiduk.
