The 19-year-old was walking on Union Street, Barnsley, when he heard footsteps running up behind him towards Sheffield Road at 11.30pm on October 14.
The man has then heard someone run across the road, before striking him across the head resulting in him falling to the floor.
He shouted: "Give me your phone and your wallet" before making off with his wallet towards Sheffield Road.
He is described as white with dark hair and approximately 6ft tall. He was wearing a blue jacket and spoke with a Barnsley accent.
Anyone with information can call police on 101 quoting incident number 1349 or Crimestoppers on 0800 555 111.