As a late caution sent the race into a two-lap overtime, Tyler Reddick held the lead - but Chastain used a deft move in the esses to reclaim the lead.
Chastain took the white flag – signalling the last lap – as the leader but Allmendinger and Bowman did not let him get away.
Coming into the last set of turns, Allmendinger got around Chastain for the lead, then Bowman went to the outside of Allmendinger and Chastain to briefly grab the lead.
At the penultimate corner, Chastain nudged Allmendinger who careened into the path of Bowman, sending both cars off course and Chastain escaped with his first career Cup Series victory in 121 starts.
The win is the first for Trackhouse Racing, which is in just its second full-time year of Cup series competition. The team is co-owned by Grammy Award-winning rapper Pitbull and former driver Justin Marks.
“It’s insane,” Chastain said. “To go up against some of the best with AJ, I mean, I know he is going to be upset with me, but we raced hard, both of us, and he owes me one.
“But when it comes to a Cup win, man, I can’t let that go down without a fight. People don’t know how good this group is. I can’t believe Justin Marks hired me to drive this car.”
Asked how he managed to win after losing control of the race on the final restart, Chastain said, “I don’t know how we got back by. I was so worried about AJ on the second-to-last restart that I let Tyler drive right by both of us.
“And AJ is so good. I’ve learned so much from him. And it was like how do you go beat the guy? He taught me so much. I’ve learned so much from so many people from 417 Speedway back home with my dad.
“I was thinking about on those late restarts, my dad used to make me race on old tyres, and back then I was not going to win. It was in my head before I even started. It crossed my mind, like, ‘We’re not going to win, we’re on old tyres’, but I couldn’t think that way."
Chastain's victory continues an impressive run in which the Floridian has not finished outside of the top three in the last four Cup races.
Bowman recovered from the final-lap skirmish with second place as Allmendinger bore the brunt of the incident, being sent into the gravel and was classified a lap down in 33rd place.
Christopher Bell was the highest-placed Toyota driver in third, beating points leader Chase Elliott to fourth place.
Reddick, whose own chances of victory had evaporated within overtime having passed Chastain and Allmendinger for the lead, finished fifth ahead of polesitter Ryan Blaney.
Blaney was beaten to stage one victory by Chastain's Trackhouse team-mate Daniel Suarez, while Denny Hamlin took the second stage victory from Kyle Busch.
Martin Truex Jr ended the day in seventh ahead of Penske rookie Austin Cindric, while Erik Jones and Austin Dillon completed the top 10.
NASCAR Cup Texas Grand Prix results - 69 laps
| Cla | Driver | Manufacturer | Time | Gap |
|---|---|---|---|---|
| 1 |
 Ross Chastain | Chevrolet | 3:20'57.006 | |
| 2 |
Alex Bowman | Chevrolet | 3:20'58.337 | 1.331 |
| 3 |
Christopher Bell | Toyota | 3:20'58.877 | 1.871 |
| 4 |
Chase Elliott | Chevrolet | 3:20'59.667 | 2.661 |
| 5 |
Tyler Reddick | Chevrolet | 3:21'00.040 | 3.034 |
| 6 |
Ryan Blaney | Ford | 3:21'00.464 | 3.458 |
| 7 |
Martin Truex Jr. | Toyota | 3:21'00.880 | 3.874 |
| 8 |
Austin Cindric | Ford | 3:21'01.003 | 3.997 |
| 9 |
Erik Jones | Chevrolet | 3:21'01.809 | 4.803 |
| 10 |
Austin Dillon | Chevrolet | 3:21'02.176 | 5.170 |
| 11 |
Kevin Harvick | Ford | 3:21'02.382 | 5.376 |
| 12 |
William Byron | Chevrolet | 3:21'02.705 | 5.699 |
| 13 |
Michael McDowell | Ford | 3:21'03.378 | 6.372 |
| 14 |
Brad Keselowski | Ford | 3:21'04.169 | 7.163 |
| 15 |
Justin Haley | Chevrolet | 3:21'04.961 | 7.955 |
| 16 |
Todd Gilliland | Ford | 3:21'05.157 | 8.151 |
| 17 |
Harrison Burton | Ford | 3:21'05.567 | 8.561 |
| 18 |
Denny Hamlin | Toyota | 3:21'06.276 | 9.270 |
| 19 |
Aric Almirola | Ford | 3:21'06.534 | 9.528 |
| 20 |
Ty Dillon | Chevrolet | 3:21'07.564 | 10.558 |
| 21 |
Chris Buescher | Ford | 3:21'07.566 | 10.560 |
| 22 |
Josh Bilicki | Chevrolet | 3:21'08.056 | 11.050 |
| 23 |
Cole Custer | Ford | 3:21'08.253 | 11.247 |
| 24 |
 Daniel Suarez | Chevrolet | 3:21'08.494 | 11.488 |
| 25 |
Kaz Grala | Chevrolet | 3:21'08.993 | 11.987 |
| 26 |
Boris Said | Ford | 3:21'12.804 | 15.798 |
| 27 |
Cody Ware | Ford | 3:21'16.641 | 19.635 |
| 28 |
Kyle Busch | Toyota | 3:21'25.221 | 28.215 |
| 29 |
Kyle Larson | Chevrolet | 3:21'39.690 | 42.684 |
| 30 |
Chase Briscoe | Ford | 3:22'46.479 | 1'49.473 |
| 31 |
Joey Logano | Ford | 3:22'51.742 | 1'54.736 |
| 32 |
Kurt Busch | Toyota | 3:23'25.217 | 2'28.211 |
| 33 |
A.J. Allmendinger | Chevrolet | 3:18'40.810 | 1 Lap |
| 34 |
 Loris Hezemans | Ford | 2:56'17.368 | 7 Laps |
| 35 |
Joey Hand | Ford | 2:53'40.502 | 9 Laps |
| 36 |
Corey Lajoie | Chevrolet | 2:40'43.746 | 12 Laps |
| 37 |
Ricky Stenhouse Jr. | Chevrolet | 2:24'30.837 | 17 Laps |
| 38 |
Bubba Wallace | Toyota | 2:08'33.994 | 25 Laps |
| 39 |
Andy Lally | Ford | 50'00.674 | 50 Laps |
| View full results | ||||
