It is believed that the 47-year-old victim was assaulted by a man at 11pm near Creams restaurant in Western Road, Brighton, on Sunday, December 2, police said.
The assault was reported to police on Monday, December 3, and police said the victim did not seek medical treatment at the time.
According to the police, the victim then became seriously ill on Saturday, December 17 and was taken to the Royal Sussex County Hospital in Brighton. At that time he was considered to be in a critical condition.
Today, the victim passed away, police said. A manslaughter investigation is now underway to establish how he died, what events led up to his death, and whether any injuries suffered in the assault caused his death.
A force spokesman said: "Anyone who may have witnessed or was involved in this incident on 2 December, or who has any information about the movements and whereabouts of the victim from 2 December to 17 December, is asked to report details online or phone 101, quoting Operation Hughes.
"Alternatively, please contact the independent charity Crimestoppers or call 0800 555 111, anonymously if you so wish, with any information you may have."