According to Hampshire and Isle of Wight Constabulary, at 4.15am on Saturday, 4 November the victim, aged 21, was walking along Clarendon Road / The Strand, at the bottom of Waverley Road in Southsea.
He was approached by two men. One of the men had a knife, and demanded the man hand over his wallet.
The victim handed it over and the offenders ran off along Clarendon Road towards the seafront.
One of the men is described as: white, around 5ft 8ins and of medium build. He had short ginger / light blond hair and was aged in his late teens. He was wearing black Nike tracksuit bottoms and a grey and black hoody. He was covering the bottom half of his face with a scarf.
The second man is described as: white, around 5ft 11ins and of skinny build. He had short dark coloured hair and was aged in his late teens. He was wearing a black hoody with the hood up and black tracksuit bottoms.
Officers would like to speak to anyone who was in the area at the time of the incident or anyone who may have any information about who is responsible.
Anyone with information is asked to report online here or call 101, quoting 44230450847.