The incident happened when the victim was walking along Mill Lane in the direction of St Vincent’s Road on Thursday night.
It was initially reported that the robbery happened just before 10.30pm but police now say that they believe it happened sometime between 9.35pm and 9.55pm.
The man was approached from behind and attacked before having his wallet stolen.
He suffered cuts to his mouth and a minor injury to his knee.
Police have said that because the victim was attacked from behind he did not see his attacker, or attackers, and they have not released a description but said the man thought they were male.
Anyone who was in the Mill Lane area around this time on Thursday night is asked to get in touch with Hampshire Constabulary.
Police would also like to hear from anyone who was driving through and have dash cam in your car with footage.
If you have any information about the incident call 101 and quote 44190372567.