The American was put into a coma and underwent emergency brain surgery after being knocked out in the 10th round of the super welterweight bout.
In a statement on his website, promoter Lou DiBella said: ”Patrick Day passed away today, succumbing to the traumatic brain injury he suffered in his fight this past Saturday, October 12, at the Wintrust Arena in Chicago, IL.
“He was surrounded by his family, close friends and members of his boxing team, including his mentor, friend and trainer Joe Higgins.”
“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 was knocked down twice prior to the stoppage and received medical support for several minutes before being taken from the ring on a stretcher.
“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.
“This is a way we can honour the legacy of Pat Day. Many people live much longer than Patrick’s 27 years, wondering if they made a difference or positively affected their world.
“This was not the case for Patrick Day when he left us. Rest in peace and power, Pat, with the angels.”