Day was knocked out in the 10th round of Saturday's super-welterweight bout in Chicago and taken from the ring on a stretcher.
Fighting on the undercard of Oleksandr Usyk's heavyweight victory over Chazz Witherspoon, Day was rushed to Northwestern Memorial Hospital and placed in a coma.
Promoter Lou DiBella announced on Wednesday that Day had died, saying: "Patrick Day passed away today, October 16, 2019, 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.
"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."
Patrick Day died today after suffering a traumatic brain injury Saturday in a knockout defeat. Statement from his promoter, Lou DiBella: pic.twitter.com/rrBxvI5ATE
— Mike Coppinger (@MikeCoppinger) October 16, 2019
Day won 17 of his 22 fights after turning professional in 2013, with four defeats and one draw.
"Patrick Day didn't need to box. He came from a good family, he was smart, educated, had good values and had other avenues available to him to earn a living," DiBella's statement read.
"He chose to box, knowing the inherent risks that every fighter faces when he or she walks into a boxing ring. Boxing is what Pat loved to do. It's how he inspired people and it was something that made him feel alive."
Fellow boxers Maxim Dadashev, Hugo Alfredo Santillan and Boris Stanchov have also passed away in 2019, each death reigniting the debate about the safety of boxing.
"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.
Devastated to hear the news of the passing of Patrick Day.I met him for the first time last Thursday,what a charming young man with a dream and a smile that lit up the room.Our deepest prayers are with his family, his trainer Joe Higgins,Charles Conwell and promoter Lou DiBella
— Eddie Hearn (@EddieHearn) October 16, 2019
"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."
Photo Getty Images Caption: Patrick Day during his bout with Charles Conwell