The announcement was made 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.
Montoya will finish out his eighth year in the car, a tenure that has produced only two wins in 241 races and finally prompted Ganassi to make a change. The car owner is convinced that Larson is the right man for the job, despite his lack of experience.
"We believe Kyle is the future of the sport," Ganassi said. "He is a unique talent."
Larson, who is of Japanese descent, came up through NASCAR's diversity program and has a sprint car background that Ganassi thinks will suit him well in the Cup series, where the cars are bigger and more powerful than those in Nationwide.
"I'm sure there will be growing pains, but I think he's ready," Ganassi said. "Some of those growing pains will come whenever his first year in the series is. But I think this is a good opportunity. My expectations and advice to him are keep the car on the track and run laps."
Larson is eighth in the Nationwide standings running for Turner Scott Motorsports, with six top-five finishes though he is still looking for his first win. He is best known for being involved in a massive crash at the end of the season-opening race in Daytona, which he got through without injury but sent debris sailing through the fencing, injuring dozens of fans.
Ganassi hopes to get Larson into a handful of Cup races over the final 12 events, probably through an arrangement with one of the lower-level teams that doesn't have a full-time driver. Next season, the youngster is set to run in both the Cup and Nationwide series, a crash course in stock car racing that should help lessen his learning curve.
"It's going to be fun year next year," Larson said. "I'm going to learn a ton."