Castroneves signed to race for Arrow McLaren SP in place of the unfit Oliver Askew at the Harvest Grand Prix in his first IndyCar start for a team other than Penske since 1999. He finished 20th in the first race and 21st in the second. Castroneves signed for Meyer Shank Racing (MSR) to compete at six races in the 2021 season, feeling the growing team would give him the opportunity to win a fourth Indianapolis 500. He had asked MSR to run enter him in more races but the request was denied. At the Indianapolis 500, Castroneves achieved his record-tying fourth career victory in the event after a late-race pass on Álex Palou.