A man suffered an injured face when he was attacked in the centre of Brighton yesterday morning (Sunday 4 September).
The 32-year-old victim, who comes from London, was taken to the Royal Sussex County Hospital in Brighton for treatment.
His attacker is suspected of taking money from him during what the robbery in Russell Square, Brighton, at about 9.45am yesterday.
Detective Sergeant Julie Greenwood, from Brighton and Hove CID, said: “We believe this is a dispute between two people who are known to each other.
“We would like to hear from anyone who may have seen the offence taking place in the street.”