The NASCAR team made the announcement Friday at Atlanta Motor Speedway, where the 21-year-old Larson will compete this weekend in a Nationwide Series race.
EGR dumped Montoya after only two wins in eight Sprint Cup seasons. The former Indianapolis 500 winner and Formula One driver is 22nd in the Cup standings.
Larson ranks eighth in the Nationwide standings as a rookie. He has never competed in a Cup race, but team owner Chip Ganassi hopes to get the youngster some experience in the final 12 races of the season.
Ganassi said he knows there are questions about Larson's lack of experience but describes him as the "future of the sport" and a "unique talent."