Mike Towell faced off Thursday night with Welsh boxer Dale Evans in the British welterweight title final eliminator.
After the match, Evans tweeted that his thoughts and prayers were with his opponent.
But Friday, the St. Andrew's Sporting Club in Glasgow said Towell had died but did not specify the cause.
A Facebook post from the club said: "Mike collapsed in the ring at the Club after he was stopped in the fifth round and was rushed to hospital after receiving attention from the ringside medical team.
"Sadly 'Iron' Mike -- who was 25 -- could not recover from his injuries and died peacefully late on Friday night with his family at his bedside."
The club described the fighter, from Dundee, as "one of Scotland's most exciting boxing talents."
Chloe Ross, Towell's girlfriend of eight years and the mother of his young son, posted a heartfelt tribute on Facebook.
"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," Ross wrote.
"It has been the longest 24 hours of our lives. My baby has lost his daddy. But he will be so so proud of his dad in what he achieved."
Towell battled to live, even after being taken off life support, she said. "He managed 12 hours, 12 whole rounds off his life support, he fought right to the end. ..."
In a statement released through the club on Twitter, Evans voiced his sadness over what happened.
"I am devastated and all my thoughts are with Mike and his family. The ingredients were there for an entertaining fight and it was that, we are both punchers and so sadly for Mike he came out the wrong side," he said.
"He is such a fantastic fighter, you are happy to win but all I wanted to do was get the victory. Not this. I never wanted to see my opponent being stretchered out of the ring.
"Who would ever want that sight? That was so hard for his family and my heart goes out to them."
Boxing fans took to Twitter to pay tribute to Towell, using the hashtag RIPIronMike.
Chris Jenkins, whose Twitter bio identifies himself as a boxer, called Towell a "true warrior in and out of the ring."
Another boxer, Anthony Fowler, tweeted, "What a tragedy it is when a man loses his life competing in the sport he loves."