Carlin will bring Sage Karam into its IndyCar line-up to replace Red Bull junior Patricio O'Ward in Toronto next weekend.
Karam's appearance will be his first IndyCar start on a road or street course since his season with Ganassi in 2015.
O'Ward is competing in the Fuji Super Formula round for Team Mugen, having been given that role by backer Red Bull following its decision to release Dan Ticktum from its junior programme.
Karam, like O'Ward a past Indy Lights champion, last raced an IndyCar for Dreyer & Reinbold Racing at this year's Indy 500, in what has become a regular tie-up.
He joins Max Chilton at Carlin for Toronto.
"We've obviously been following Sage's career closely since he won the Indy Lights championship in 2013 and he's done a really great job," said team boss Trevor Carlin.
"I think he'll be a great addition to the team and a good team-mate for Max."
Karam said he was "extremely excited and grateful to get back behind the wheel on a road course again for the first time since 2015".
The 24-year-old may remain in the car for Iowa Speedway, where he scored his only IndyCar podium in 2015 (pictured above), but would likely be partnering Conor Daly there as Daly is expected to be Chilton's oval replacement once more.
O'Ward's part-time IndyCar programme with Carlin also included the Mid-Ohio and Portland rounds later in the year.
- O'Ward's rapid rise
- The driver thrust up Red Bull's junior queue
Neither clashes with Super Formula, but O'Ward has admitted to uncertainty over the rest of his 2019 schedule now he is under Red Bull's wing and aiming for a Formula 1 future.
He made his Formula 2 debut at the Red Bull Ring last weekend to stand in for the banned Mahaveer Raghunathan at MP Motorsport.