Sixth-ranked Spieth, the 2015 Masters and US Open champion, fired a final-round par 70 to finish 72 holes level with Berger.
Sixth-ranked Spieth, the 2015 Masters and US Open champion, fired a final-round par 70 to finish 72 holes level with Berger on 12-under 268 at TPC River Highlands near Hartford, Connecticut.
On the par-4 18th hole in the playoff, Spieth hit a tree off the tee but the ball bounded into the fairway. His approach went into a greenside bunker. But then he blasted out and the ball bounced twice on the green and rolled three feet into the cup.
"For the bunker shot to go in, that was awesome," Spieth said. "I don't know if I'll ever have a moment like that again."
Spieth helped quiet a cheering crowd as Berger had a long putt from off the green to extend the playoff but Berger could not match Spieth's stunning feat and the celebration began in earnest moments later.
"It's just Jordan doing Jordan things," Berger said. "I played great today. Obviously I wanted to win. I'm just speechless right now."
Spieth, 23, won earlier this year at Pebble Beach and after completing a wire-to-wire victory in his Travelers debut became the second-youngest player to win 10 PGA titles, trailing only Tiger Woods.
"That's incredible," Spieth said. "It feels like I've been out here a long time. I feel very lucky."
Berger, who had been five back of Spieth at one stage, led last year's Travelers by three strokes after 54 holes only to lose.
New Zealand's Danny Lee and American Charley Hoffman shared third on 270, one stroke ahead of England's Paul Casey and Americans Patrick Reed and Boo Weekley, who managed his first top-25 finish of the season.
Deadlocked with Berger at 11-under down the final holes of the round, Spieth sank a 16-foot birdie putt at the par-4 15th hole to reclaim a one-stroke edge.
Berger answered with a seven-foot birdie putt at the par-4 17th to match Spieth on 12-under.
Both players parred 18 to set up the playoff, each finding a greenside bunker then blasting out to three feet and making the par putt.
"It was a great up and down in regulation," Spieth said. "It was a battle. Got off to a great start. The putter let me down most of the round."
Spieth opened with back-to-back birdies, dropping his approaches five and seven feet from the cups, but took a bogey at the fourth and needed an 11-foot putt to rescue par at the sixth and stay at 13-under.
Spieth stumbled again at the 12th, missing a seven-foot par putt to fall to 12-under, then lipped out a four-foot birdie putt before parring 13.
Berger sank a five-foot birdie putt at the par-4 15th to reach 11-under and when Spieth missed a three-foot par putt to bogey 14, the Americans were deadlocked atop the leaderboard at 11-under, setting up the final drama.
Third-ranked Rory McIlroy shot 64 to finish on 274, having struggled much of the week before rebounding in the final round to carry some confidence into the run-up to next month's British Open.