The victim was walking on Kennington Park Road, near the junction for Othello Close, when the thief struck.
The man was described as being black, aged between 25 and 35, and of large build with short cropped hair.
He is thought to have been wearing a baggy t-shirt and dark trousers, and was riding a racer-style bike with a silver dropped handlebars.
He fled along the pavement before turning left near to the entrance of Kennington Underground Station, onto Braganza Street after the robbery at 10.50pm on July 12.
The victim wasn’t injured and no arrests have been made.
Police have released an image of a man they would like help identifying as part of their investigation.
DC John Edwards, from the local policing team in Lambeth, said: “We have carried out a number of enquiries and we are now asking for the public’s help to name this man. I would urge anyone who recognises him to contact us immediately.”
Anyone who can help the investigation is asked to call 101 quoting CAD 2509/07Sep.
To remain anonymous contact Crimestoppers on 0800 555 111.