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 Ryan Newman left Larson and Chastain to battle side-by-side on the restart with 13 laps to go.
Larson nudged Chastain up into the wall as Joey Logano hit Martin Truex Jr., which ended Truex’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 to earn his third win of the 2023 season.
The victory is the seventh of Byron’s career and the 100th win for Hendrick Motorsports’ No. 24 Chevrolet.
“It’s pretty amazing. My granddad passed away on Thursday, and just man, I wish my family could be here today. Just things have a way of working out, honestly,” an emotional Byron said after the race. “It just worked out that way today.
“We didn’t have the best third stage. We just kept battling, and things just kind of come back around. Want to wish Happy Mother’s Day to my mom. My sister just graduated school, so big day.
“I definitely didn’t expect this. But just thankful for a great team, and yeah, just things have a way of working out, and come back here to Darlington and have it go exactly the other way.”
One year ago at this race, Byron appeared to have a victory in hand only to have Joey Logano nudge him out of the way to claim the win.
Chase Elliott wound up third on Sunday, Brad Keselowski fourth and Bubba Wallace rebounded from a bad pit stop earlier in the race to finish fifth.
Completing the top-10 were Harrison Burton, Kyle Busch, Justin Haley, Ryan Blaney and Chris Buescher.
Stage 1
Truex led all but one of the first 90 laps and easily held off Byron by more than three seconds for the Stage 1 win.
Wallace was third, Chastain fourth and Busch rounded out the top five.
Daniel Suarez was penalized for speeding on pit road during the race’s first round of stops under caution and had to restart from the rear of the field. Larson’s No. 5 Chevrolet fell off the jack during his stop and he restarted 25th.
Stage 2
Truex wrecked and spun on the final lap while trying to chase down Chastain for the lead. That placed the race under caution and allowed Chastain to take the Stage 2 win.
Busch ended up second, Larson third, Byron fourth and Keselowski completed the top five.
Following a round of green-flag pit stops, Chastain ran down Truex and passed him on the inside entering Turn 3 on lap 151 to take the lead for the first time in the race.
Stage 3
Following the break between Stages 2 and 3, the lead-lap cars all pit with Larson the first off pit road. When the race resumed on lap 194, Larson was followed by Chastain and Busch.
As the field came off Turn 2 on the restart, the right-rear tire came off of Erik Jones’ No. 43 which triggered a multi-car pile-up on the backstretch. Among those collected were Austin Dillon, Austin Cindric, Newman and Suarez.
While trying to catch Chastain for the lead on lap 219, Busch hit the outside wall and lost several positions on the track.
With 50 laps remaining, Byron had cut Chastain’s lead down to half-a-second as teams prepared for a final round of green flag pit stops to take new tires and fuel to make it to the finish.
Once the cycle of stops was completed, Larson reclaimed the lead with 30 laps remaining.
Newman wrecked off Turn 4 late in the race which sent all the lead-lap cars down pit road for new tires. Bell, who came off second, was forced to pit again for a possible loose wheel. On the restart with 13 laps remaining, Larson led Chastain, Logano, Truex and Elliott.
As the field entered Turn 2 on the restart, Logano got into Truex which triggered a multi-car accident. Tyler Reddick, Harvick, Aric Almirola, Elliott and Busch were among those caught up in the incident.
Chastain was ahead of Larson when the caution was displayed and led the way on the restart with six laps remaining. Chastain and Larson wrecked on the restart which allowed Byron to inherit the lead as the race went into a two-lap overtime.
