Bidding for his first PGA Tour win in his 159th start, the English Ryder Cup star (34) had a two-shot lead with three holes to play and a one-shot lead over Bradley heading down the 18th, but misjudged the wind and came up just short of the green with his approach.
Bradley fired his second to six feet, but Fleetwood left his 50-footer seven feet short and missed for par from just behind the American’s ball marker.
Having shown his rival the line, he watched Bradley roll in his birdie putt to win the title for the second time in three years and take a massive step towards qualifying automatically for his own Ryder Cup team.
“Pretty gutted right now,” Fleetwood said after his closing 72. “A couple of wrong clubs down the last couple. Obviously didn’t putt great today.
“Probably the worst way to finish, leading by two with three to play, and leading by one going down last and you don’t even make it to a playoff.
“So it’s kind of the worst way that it could go. But Keegan did great down the closing stretch. He holed putts, hit great shots. Gutted really so we’ll just go home, start practising again.”
He led by three strokes overnight but walked off the fourth green tied for the lead with Bradley after making three bogeys and a birdie in his first four holes.
He added: “I haven’t been in this situation for a while…. I’m upset now, I’m angry — when it calms down, I’l look at the things that I did well, look at the things that I can learn from.
“Right now I would love to just go and sulk somewhere and maybe I will do, but there’s just no point making it a negative for the future really, just take the positives and move on.
“I obviously played great, I put myself in a great position, I was leading the tournament for 71 holes.
“I just want to make sure that I can put myself in this position as soon as possible again and try and correct what I did this time.”
Bradley was understandably thrilled to claim his eighth PGA Tour win in front of a home crowd.
“Absolutely incredible,” Bradley said after a 68 gave him a one-shot win on 15-under over Fleetwood and Russell Henley, who chipped in for birdie from greenside rough at the 18th.
“I feel an obligation to play for the people of New England and the northeast and represent them, and this is the best way I can.”
Bradley made a series of long putts throughout the day but it was his final putt that meant the most to him.
“Of all the shots and all the putts I’ve hit, I think I’ll remember that one the most,” he said.
Asked if he could be a useful playing member of his Ryder Cup team, he added: “Go USA.”
Rory McIlroy made six birdies in a final round 65 to tie for sixth on 12-under with world number one Scottie Scheffler, who also shot five-under.
Like Shane Lowry, whose 69 left him tied for 45th on one-under-par, McIlroy heads home to prepare for The Open at Royal Portrush.
“It was a good way to end the week,” McIlroy said. “Good, positive vibes going into a couple-week break here and get ready for Scotland and The Open Championship.
“But, yeah, definitely saw some positive signs in the game overall, which was really good to see.
“Looking forward to getting back over to my neck of the woods for a couple of weeks and be prepared for a little bit of links golf.”