The incident occurred when the victim, in his 50s, was approached by three youngsters who asked him for tobacco in Havant Park yesterday morning, between 11am and 11.20am.
When he handed some over, the group grabbed him – taking his wallet and mobile phone.
Police have launched an investigation to try and track down the suspects.
One was white, stocky, and had short, dark hair and was wearing a blue Lonsdale hoodie when the robbery occurred.
Another suspect was white, of a medium to stocky build and wore dark clothing.
Officers – who are particularly concerned to hear from anyone who knows of the direction the youths ran – have urged witnesses to contact them for their enquiries.
Call police on 101 quoting the crime reference number 44180210147, or call Crimestoppers anonymously on 0800 555 111.