Reddick got through Turn 1 on the restart unscathed while most of the field did not, including Ross Chastain, who ran off onto the access road and returned to the track in the lead.
With multiple cars going off track battling for position, Reddick eventually ran Chastain down and passed him for the lead and held on for the victory.
Chastain, however, was penalised by NASCAR for cutting the corner and dropped to 27th place.
The win is the second of Reddick’s career with both coming this season and both on road courses.
“I was like, ‘uh-oh’ – but that was a scenario that had been talked about,” Reddick said of Chastain’s move.
“If you get bottled up, what do you do? Take the access road.
“I couldn’t believe he got ahead of me. I was kind of waiting to see if he was going to have a penalty because I didn’t want to move him out of the way and make his race worse than what it was.
“Yeah, I was really surprised by that, but hey, we made it work. Hats off to Ross for trying to do that, but really glad it didn’t end up working out because I’d have been pretty [upset].”
Sunday’s win comes after Reddick announced a few weeks ago that he would be leaving Richard Childress Racing after the 2023 season to join 23XI Racing.
Asked how his team has been able to remain focused on winning, Reddick said: “We just know what we’re capable of, and we did that at Road America.
“Certainly it was a little bump in the road, but we went out and won a race fair and square a couple weeks ago and, if we change nothing, we keep working really, really hard, we find a way back to Victory Lane.
“Just really glad to be able to do it here in Indianapolis. This is one really special place to race, and really excited to kiss the bricks.”
Austin Cindric, Team Penske, Discount Tire Ford Mustang
Photo by: Matthew T. Thacker / NKP / Motorsport Images
With the revised results, Austin Cindric was credited with second, Harrison Burton a career-best third, Todd Gilliland fourth and Bubba Wallace fifth.
Completing the top 10 were Joey Logano, AJ Allmendinger, Michael McDowell, Cole Custer and Chris Buescher, who at one point was two laps down early in the race.
It was Christopher Bell who had lead at the start of stage 3, having not pitted in the break between stages.
On Lap 51, Reddick finally got around Bell to retake the lead and he continued to run first after a caution period when Kyle Larson lost his brakes at Turn 1 and viciously slammed into Ty Dillon.
With 10 laps to go, Reddick’s lead over Ryan Blaney improved to 2.9 seconds as Chase Elliott ran third, 3.3s behind the leader.
There was then another caution in the closing laps when the front-right fender from Bell’s Toyota flew off and there was chaos behind Reddick on the restart as the field went five-wide, sending multiple cars spinning and off the track.
Austin Dillon ended up stuck in the gravel which is what brought out the caution and sent the race into overtime.
Elsewhere, former Formula 1 driver Daniil Kvyat – who was making his Cup series debut – had a disappointing race as he came to a stop off track after his Ford lost power on lap 32.
Result – 86 laps: