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.
