Maxim Dadashev was beaten in 11 rounds by undefeated Subriel Matias in Maryland in an IBF light-welterweight title eliminator match.
The 28-year-old, from Russia, suffered a subdural haematoma which caused him to collapse in front of fans.
He then left the arena after struggling to walk out of the ring.
It was said he vomited and then lost consciousness as his team attempted to carry him backstage.
Russia media now reports the fighter has now died after a long battle by doctors to save his life.
Dadashev’s wife – with whom he has a young son – had travelled from St Petersburg to be with her husband in Washington.
He had 14 fights in a professional ring and scored 13 victories –including 11 by KO.
The fighter was a silver and two-time bronze medalist in the championships of Russia.
He reportedly underwent brain surgery before he died today following the fight on July 20.
Top boxing promoter Eddie Hearn said:
“So terribly sad to hear the news of the passing of Maxim Dadashev.”
His trainer Buddy McGirt pulled Dadashev out of the fight as he took heavy punishment during the 11th round against Matias.
After the fight, McGirt said:
“God forbid, one punch as you know can change a whole guy’s life and I wasn’t going to let that happen.
“So I’d rather have them be mad at me for a day or two then to be mad at me for the rest of their life.”
While reacting to the death of Dadashev on Tuesday, McGirt to ESPN:
“It just makes you realise what type of sport we’re in, man…He did everything right in training, no problems, no nothing. My mind is like really running crazy, right now. Like what could I have done differently? But at the end of the day, everything was fine [in training]. He seemed OK, he was ready, but it’s the sport that we’re in. It just takes one punch, man.”
According to Mary IH Cobb, the neurosurgeon who operated on Dadashev, the fighter had suffered a brain bleed on his right side.
They removed part of his skull in bid to remove the swelling, and gave him medication.
Cobb had said she expected the boxer to remain in hospital for several months – describing him as “critical”.
Dadashev took numerous heavy shots to the side of the face in the fight before McGrit threw in the towel in the 11th.
He was also left unable to swallow water between rounds as the fight rolled on. After his win, Matias said:
“I hope that Maxim is all right. He is a great fighter and a warrior.”
Dadashev was born in St Petersburg but was based in California. He was undefeated in 13 fights going into Friday’s bout. He was married with one child.