Thames Valley Police is releasing an E-Fit image in connection with a robbery in Milton Keynes.
The victim was walking in an underpass that runs underneath the redway between Grafton Street and Kirkstall Place, Oldbrook, at about 10pm on 12 February when a man shouted out to him and approached him.
The man threatened the victim with a knife and demanded that he handover his mobile phone and wallet, which contained cash and a bank card.
The victim, a 25-year-old man, handed the items over and the offender then told him to walk with him along Kirkstall Place.
The offender became nervous when members of public walked past nearby and put away the knife.
The victim asked for his belongings back. The offender gave him back his mobile phone and bank card but kept the cash, before making the victim wipe the card to remove any fingerprints.
The offender told the victim not to call the police and then ran off towards Douglas Place.
The victim was not injured.
The offender was a white man aged 20 to 30, with a skinny build, who was about 5ft 8ins tall. He had green eyes and a beard that was golden brown in colour.
He was wearing a grey top with the hood up, pulled tightly around his face.
The offender was carrying a dark-glassed bottle with a red label in his left hand and a knife with a black handle and silver blade in his right hand.
Investigating officer DC Damien Freeman, of the Investigation Hub at Milton Keynes, said: “We would like to speak to the man in the E-Fit image in connection with our investigation.
“We are asking the man, or anyone who recognises him and has information that could help us to locate him, to call the Thames Valley Police non-emergency number on 101, quoting reference number 43180045938.
“We know that at least two members of the public walked past the offender and victim during the incident and we are also appealing for them and any other witnesses to get in touch.”