Stewart was leading a 30-lap Front Row Challenge at the Southern Iowa Speedway when, with five laps to go, a car driven by Josh Higday spun on Turn 4. The three race leaders -- Stewart, Tasker Phillips and Tony Shilling -- were involved in the subsequent crash.
Stewart was reportedly talking with officials as he left the track on a stretcher. There apparently were no other injuries.
Stewart was taken to a local hospital and underwent surgery to set the broken fibula and tibia in his right leg. It isn't known how long he will be out of racing but Stewart will miss this weekend's NASCAR race at Watkins Glen, N.Y.
The Stewart-Haas Racing team is yet to announce a replacement driver for the race.
Stewart is 11th in the Sprint Cup drivers' standing this year with one win, a victory June 2 at Dover, Del. He won the drivers' championship in 2002, '05 and '11. He has won 48 NASCAR races in his career, which is 13th most all-time.