Police were called just after 5.30pm on Friday to reports of a man being attacked with a knife on Arundel Street, at the junction with Buckingham Street.
On arrival, a 19-year-old man from Fratton was located with a stab wound. He was taken to hospital where he is being treated for his injuries, they are not considered life-threatening at this time.
A 20-year-old man and an 18-year-old man, both from Portsmouth, have been arrested on suspicion of assisting an offender. They have been released from custody but remain under investigation.
Officers are now releasing CCTV image of a man they wish to speak with in connection to this investigation.
He is described as a white man, approximately 6ft tall with a slim build. He was wearing a black puffa jacket and a dark blue hood, dark coloured jogging bottoms, with black trainers and white socks.
He was also in possession of a black scooter with green writing on the stem.
Police appreciate there is no image showing his face, but are hopeful that someone who knows him will recognise the clothing.
Anyone with information is asked to phone 101 with reference 44210165339. Alternatively, you can call Crimestoppers 100% anonymously on 0800 555 111.