The 27-year-old underwent brain surgery after he was knocked out in the ring last Saturday. He did not recover.
Promoter Lou DiBella says Day died at Northwestern Memorial Hospital on Wednesday.
“He was surrounded by his family, close friends and members of his boxing team, including his mentor, friend and trainer Joe Higgins,” DiBella said in a statement on his website. “He was a son, brother and good friend to many. Pat’s kindness, positivity and generosity of spirit made a lasting impression with everyone he met.”
Conwell, Day’s opponent, has been posting messages of concern on social media since the fight.
“I never meant for this to happen to you,” he wrote in an open letter to Day and his family on Monday before the boxer’s death. “All I ever wanted to do was win. If I could take it all back I would. No one deserves for this to happen to them.
“I replay the fight over and over in my head, thinking what if this never happened, and why did it happen to you?”
Conwell, 21, is a former Olympian with an 11-0 record and eight knockouts.
He defeated Day at Wintrust Arena on Saturday night. Conwell knocked Day down twice in the fight before delivering the knockout blow in the 10th round of their junior middleweight bout.
Conwell hit Day with a left hook to knock him out. Day’s head slammed into the canvas, and the referee stopped the fight without a count.
Day was carried out on a stretcher. He reportedly suffered a seizure and fell into a coma after the knockout. He never regained consciousness.
The native of Freeport, N.Y., had a career record of 17-4-1 with six knockouts. He also had a successful amateur career with two national titles and won the New York Golden Gloves tournament. He was a U.S. Olympic team alternate in 2012.
Day learned to box from his neighbour, Higgins, a former New York City firefighter who lost his brother in the World Trade Center attacks, according to ESPN.
Higgins has credited Day with helping him get through the grief of losing his brother on Sept. 11, 2001.
“Pat saved my life,” Higgins told ESPN earlier this week. “I’m dying … I feel like I’m responsible, like I let him down. My special kid.”
Day is the third professional boxer to die after suffering a traumatic brain injury in the ring this year.
Russian boxer Maxim Dadashev died at the age of 28 in July, four days after a fight in Maryland. Alfredo Santillan, a 23-year-old from Argentina, died a few days later after a separate bout in his home country.
DiBella suggested in his statement that Day’s death should inspire boxing officials to make the sport “safer for all who participate.”
—With files from Reuters and the Associated Press