Larson emerged late in the race with the lead following the final round of green-flag pit stops, but a caution for a spin by series returnee Ryan Newman left Larson and Chastain to battle side-by-side on the restart with 13 laps to go.
Hendrick Chevrolet driver Larson nudged Trackhouse counterpart Chastain up into the wall as behind Joey Logano hit Martin Truex Jr., which ended the stage one winner's day and put the race back under caution.
On the ensuing restart with six laps remaining, Chastain and Larson again lined up alongside each other and this time it was Chastain who nudged Larson up the track, which wrecked both cars.
Byron inherited the lead from the melee and then had little trouble holding off Kevin Harvick in the two-lap overtime for the seventh win of his career and the 100th for Hendrick’s #24 Chevrolet.
An emotional Byron, whose grandfather passed away on Thursday, admitted afterwards that “we didn’t have the best third stage” after finishing second to Truex in the opening stage.
His victory was extra special coming one year after he was nudged out of a likely win by Logano.
“We just kept battling, and things just kind of come back around,” he said. “I definitely didn’t expect this.”
Chase Elliott ended up third ahead of Brad Keselowski. and Bubba Wallace, who rebounded from a bad pit stop earlier in the race to finish fifth.
Harrison Burton, Kyle Busch, Justin Haley, Ryan Blaney and Chris Buescher completed the top 10.
Chastain had taken the Stage 2 win, running ahead of Truex when the Joe Gibbs Racing driver's final lap spin put the race under caution.
But when racing resumed for Stage 3, Larson exited the pits ahead of Chastain and held the upper hand until their dramatic on-track clashes.
At the finish Larson was classified 20th, Chastain 29th.
NASCAR Cup Darlington Race Results (295 laps)
| Cla | # | Driver | Manufacturer | Time | Gap |
|---|---|---|---|---|---|
| 1 | 24 |
 William Byron |
Chevrolet | 3:23'23.274 | |
| 2 | 4 |
Kevin Harvick |
Ford | 3:23'24.055 | 0.781 |
| 3 | 9 |
Chase Elliott |
Chevrolet | 3:23'24.754 | 1.480 |
| 4 | 6 |
Brad Keselowski |
Ford | 3:23'25.072 | 1.798 |
| 5 | 23 |
Bubba Wallace |
Toyota | 3:23'25.222 | 1.948 |
| 6 | 21 |
Harrison Burton |
Ford | 3:23'25.283 | 2.009 |
| 7 | 8 |
Kyle Busch |
Chevrolet | 3:23'25.499 | 2.225 |
| 8 | 31 |
Justin Haley |
Chevrolet | 3:23'25.923 | 2.649 |
| 9 | 12 |
Ryan Blaney |
Ford | 3:23'26.507 | 3.233 |
| 10 | 17 |
Chris Buescher |
Ford | 3:23'26.517 | 3.243 |
| 11 | 38 |
Todd Gilliland |
Ford | 3:23'26.599 | 3.325 |
| 12 | 11 |
Denny Hamlin |
Toyota | 3:23'26.634 | 3.360 |
| 13 | 47 |
Ricky Stenhouse Jr. |
Chevrolet | 3:23'26.713 | 3.439 |
| 14 | 20 |
Christopher Bell |
Toyota | 3:23'26.905 | 3.631 |
| 15 | 41 |
Ryan Preece |
Ford | 3:23'27.005 | 3.731 |
| 16 | 54 | Ty Gibbs | Toyota | 3:23'27.378 | 4.104 |
| 17 | 14 |
Chase Briscoe |
Ford | 3:23'27.447 | 4.173 |
| 18 | 22 |
Joey Logano |
Ford | 3:23'28.089 | 4.815 |
| 19 | 2 |
Austin Cindric |
Ford | 3:23'28.205 | 4.931 |
| 20 | 5 |
Kyle Larson |
Chevrolet | 3:23'30.686 | 7.412 |
| 21 | 10 |
Aric Almirola |
Ford | 3:23'32.240 | 8.966 |
| 22 | 45 |
Tyler Reddick |
Toyota | 3:23'53.853 | 30.579 |
| 23 | 16 |
A.J. Allmendinger |
Chevrolet | 3:23'32.881 | 1 Lap |
| 24 | 7 |
Corey Lajoie |
Chevrolet | 3:23'27.688 | 2 Laps |
| 25 | 43 |
Erik Jones |
Chevrolet | 3:23'27.922 | 2 Laps |
| 26 | 42 |
Noah Gragson |
Chevrolet | 3:23'28.271 | 2 Laps |
| 27 | 77 |
Ty Dillon |
Chevrolet | 3:23'28.525 | 2 Laps |
| 28 | 51 |
Ryan Newman |
Ford | 3:23'33.173 | 4 Laps |
| 29 | 1 |
Ross Chastain |
Chevrolet | 3:13'52.919 | 8 Laps |
| 30 | 48 | Josh Berry | Chevrolet | 3:23'33.662 | 9 Laps |
| 31 | 19 |
Martin Truex Jr. |
Toyota | 3:03'25.799 | 15 Laps |
| 32 | 78 |
B.J. McLeod |
Chevrolet | 2:39'01.176 | 69 Laps |
| 33 | 34 |
Michael McDowell |
Ford | 2:14'42.377 | 95 Laps |
| 34 | 99 |
 Daniel Suarez |
Chevrolet | 2:12'27.375 | 96 Laps |
| 35 | 3 |
Austin Dillon |
Chevrolet | 2:11'42.064 | 99 Laps |
| 36 | 15 |
Brennan Poole |
Ford | 24'24.050 | 256 Laps |
| View full results | |||||
