Mike Towell suffered a fifth-round loss to Dale Evans in a St Andrews Sporting Club fight in Glasgow on Thursday night and was rushed to Queen Elizabeth University Hospital.
His management said he passed away with his family at his bedside.
Towell's partner, Chloe Ross, said he "fought right to the end".
"I'm absolutely heartbroken to say my annoying best friend passed away tonight at 11.02 very peacefully," she wrote.
"Michael had severe bleeding and swelling to his brain. He had been complaining of headaches for the last few weeks but we put it down to migraines with the stress of his fight.
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Ross added: "Once he was taken off of his life support he managed 12 hours, 12 whole rounds off his life support, he fought right to the end and he's done us all so proud... He left doing the thing he's best at... There's only one."
Trainer Shane McGuigan said on Twitter: "Heartbreaking news that Mike Towell has lost his life. May he rest in peace, thoughts go out to his loved ones. #RIP".
Towell was knocked down in the first round of the fight, before recovering to continue the match.