Police said the 45-year-old was attacked on the pavement outside Halifax Bank, on Old Market Place, Grimsby, at around 4.30pm yesterday.
He was taken to Diana, Princess of Wales Hospital for treatment and died there this morning. The cause of death is yet to be established.
Humberside Police released a description of a man they want to speak to in connection with the assault.
They said he was white, aged between 20 and 45, with dark rimmed glasses. He wore a beige or grey waist length jacket with a peaked hood, which was up, dark trousers and grey or brown trainers.
Det Insp Stewart Miller said: “At the current time we believe this was an isolated incident and there is no risk to the wider community.
“Our initial investigations have lead us to believe this attack was not targeted at any specific section of the community and will be increasing patrols in the area to provide reassurance to people living and working in the area.”
Witnesses are asked to call the police incident room on 01472 290204 or ring 101, quoting log number 372 of January 15.