American boxer, Patrick Day died four days after suffering head injuries in a fight against compatriot, Charles Conwell.
Day was put in a coma with a serious brain injury at the weekend after he was knocked out in the 10th round of his super welterweight bout in Chicago.
Promoter Lou DiBella confirmed Day had died on Wednesday.
Day, aged 27, won 17 of his 22 fights, with four defeats and one draw
His last opponent, Cornwell did write an emotional letter after the fight, as the 21-year-old wished it all never happened.
“I never meant this to happen to you, all I wanted to do was win,” he said.
“If I could take it all back, I would. No-one deserves this to happen to them.”