Magnussen will be joining a grid that features his former Haas team-mate Romain Grosjean, ex-Sauber man Marcus Ericsson and two-time Indy 500 winner Takuma Sato.
The Dane, who will race at Le Mans in a Peugeot HyperCar next year, told Motor Sport last February of his burning ambition to compete in IndyCar series, with a particular eye on the Brickyard.
“Ovals are something I’ve always wanted to try, just going 230 miles an hour – I feel like it’s exciting. The Indy 500 is a race that I just need to be doing one day.”
Magnussen is subbing for an IndyCar team on the up. The squad has won two races with Pato O’Ward this season, most recently the second Detroit round the day after Rosenqvist’s crash.