Wayne Amess, 40, is believed to have been an innocent bystander when a fight broke out near The George pub on the corner of Walton Street and Spring Bank West, at about 2.30am on New Year's Day.
He was punched in the face and fell unconscious on the pavement.
Mr Amess, of Woodgate Road in the city, was taken by ambulance to Hull Royal Infirmary in a critical condition with a serious head injury, but died just before 8pm that day.
A woman aged 38 from Hull, and an 18-year-old man, also from the city, were arrested in connection with the incident and were being questioned by detectives at separate police stations in Hull yesterday.
Police believe there was an altercation inside the pub earlier in the evening and have appealed for witnesses to either incident to come forward.
Det Chief Insp John Harding, of Humberside Police, said: "The victim has sadly died and this is now a murder inquiry.
"There were a lot of people at the pub at the time of the incident and it is important that we speak to everyone who was in the area and saw what happened.
"We believe there may also have been an altercation inside the pub earlier in the evening and I would like to speak to anyone who was there and witnessed what happened – both inside and outside the venue."
Officers are also examining footage from closed-circuit television cameras in the area.
Anyone with information is asked to call the Major Incident Team at Tower Grange Police Station on (01482) 796275.