Day, 27, was taken to Northwestern Memorial Hospital in Chicago following his Saturday fight with Charles Conwell on the undercard of Oleksandr Usyk's victorious heavyweight debut against Chazz Witherspoon.
"On behalf of Patrick's family, team, and those closest to him, we are grateful for the prayers, expressions of support and outpouring of love for Pat that have been so obvious since his injury," promoter Lou DiBella said in a statement. "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."
Day (17-4-1, 6 KOs) struggled in his fight with Conwell, a 2016 U.S. Olympian from Cleveland who improved to 11-0 with his win over the Freeport, N.Y. fighter.
Day was knocked down in the fourth and eighth rounds during the 10-round bout. In the final round, Conwell caught Day with a left hook that floored him, causing his head to slam against the canvas as he fell onto his back.
Medical personnel rushed into the ring once the fight was stopped by the referee and attended to Day, who never regained consciousness and was led out on a stretcher. He suffered a seizure in an ambulance while en route to the hospital and eventually went into a coma after emergency brain surgery.
DAZN, the streaming platform that aired the bout, offered its condolences in a tweet.
"DAZN is incredibly saddened to learn about the passing of Patrick Day," a DAZN spokesman said in a statement. "Our heartfelt thoughts are with his family and friends during this difficult time."
On Monday, Conwell opened up about Saturday's events.
“I never meant for this to happen to you," he wrote in an Instagram caption that took the form of a letter to Day. "All I ever wanted to do was win. If I could take it all back I would [sic] 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. I can’t stop thinking about it myself I prayed for you so many times and shedded [sic] so many tears because I couldn’t even imagine how my family and friends would feel."
Prior to his loss to Conwell, Day suffered a unanimous decision loss to prospect Carlos Adames and won six fights in a row before that, according to BoxRec.
In 2012, Day was the New York Daily News Golden Glove Champion, USA Boxing welterweight national champion, and winner of the Sugar Ray Robinson Outstanding Athlete award.
Day is the third boxer to die this year as a result of injuries sustained in the ring. Maxim Dadashev, a 28-year-old Russian light welterweight, died on July 23 from injuries suffered in his junior welterweight loss four days earlier in Maryland against Puerto Rican contender Subriel Matias.
Two days later, on July 25, Argentinian fighter Hugo Alfredo Santillan died after fighting to a draw in his super lightweight bout. He collapsed at the end of the fight.
"It becomes very difficult to explain away or justify the dangers of boxing at a time like this," DiBella said. " This is not a time where edicts or pronouncements are appropriate, or the answers are readily available. It is, however, a time for a call to action. While we don't have the answers, we certainly know many of the questions, have the means to answer them, and have the opportunity to respond responsibly and accordingly and make boxing safer for all who participate."
Fox News' Stephen Sorace contributed to this report.