Robert Wickens will switch from DTM to IndyCar and drive for Schmidt Peterson Motorsports next year.
Wickens will replace Mikhail Aleshin, who was dumped by SPM four races before the end of the season.
The Canadian had been hunting for a new drive after Mercedes announced its decision to quit DTM at the end of 2018.
Wickens was already known to SPM by virtue of a car swap with friend and compatriot James Hinchcliffe early this year, and filled in for Aleshin in Friday practice at Road America in June when the Russian’s return to the USA from the Le Mans 24 Hours was delayed by visa issues.
He also almost made an early debut in place of Esteban Gutierrez after the Mexican crashed in Toronto qualifying.
Now Wickens will join Hinchcliffe as team-mate on a permanent basis after the 30-year-old signed a multi-year renewal with SPM.
“Until this season, I’d never really considered leaving Mercedes or the DTM,” said Wickens.
“I can’t thank Mercedes enough for the last six years and wish them the best of luck for the future – it’s probably been the best six years of my life.
“I’ve had a lot of ups and downs on the way but have never had so much support from a team before.
“It’s been an honour to represent the brand and maybe it’s not the last time.”
In an announcement made by Mercedes, the manufacturer said “the opportunity to move came at exactly the right time” for Wickens, following its decision to withdraw from the DTM.
Mercedes DTM boss Ulrich Fritz said, “It’s obviously a big pity that Rob is leaving us. We’re losing both a great driver and a terrific person.
“Rob has been an important member of our team over the past six years.
“Although his departure obviously makes us feel sad, we are, nevertheless, pleased for him.
“The IndyCar Series will definitely be a great experience for Rob, and I’m sure he will put his stamp on this series as well.”
Wickens added, “It’s going to be a very steep learning curve. A lot of things will be completely new to me. I’ve never driven on an oval before.
“As everyone knows, I haven’t been driving formula cars over the last six years since I’ve been in DTM, so I have to get up to speed very quickly but am looking forward to the new challenge.”
The 2018 Verizon IndyCar Series starts in St Petersburg in March.