A teenager had £80 stolen at knifepoint as he was walking home.
The 18-year-old Spaniard was returning to Southsea in Montgomerie Road early on Saturday morning when he was confronted by two men.
He was threatened with a knife and had the cash taken from him but was not injured in the incident between 1.20am and 1.30am.
The first suspect was white, in his early 20s, of average build, with dark hair and was wearing a grey jumper, dark green T-shirt, sports trousers and trainers.
The second suspect was black, aged 25 to 30 years old, muscular, with no facial hair, and was wearing a light grey hooded top with the hood up and dark trousers.
Anyone who saw the robbery or who has information about the identity of the suspects is asked to contact Fratton Police Station on 101.