The American was hospitalised on Sunday after being knocked out by his 21-year-old opponent at Chicago’s Wintrust Arena.
Day required medical attention in the ring and was fitted with a neck brace and oxygen mask before being taken away on a stretcher.
The super welterweight, a two-time golden gloves champion, was put in a medically induced coma by doctors after the 10th round knockout, but tragically passed away on Wednesday.
Tributes have come flooding in for the fighter, with his promoter Lou DiBella confirming Day had died “surrounded by his family, close friends and members of his boxing team.”
Day was knocked down twice by Conwell in the fourth and eighth rounds before the fight ended in the 10th.
The 27-year-old was immediately taken to hospital and reportedly suffered seizures en route.
After the knockout, DiBella entertainment said in a statement: “Patrick Day suffered a traumatic brain injury during his bout on Saturday night.
“He was rushed to Northwestern Memorial Hospital where he underwent emergency surgery.
“As of Sunday evening, Patrick is in a coma caused by the injury and is in extremely critical condition.
“On behalf of Patrick’s team, we appreciate the outpouring of support, prayers, and offers of assistance from all corners of the boxing community.”