The 18-year-old man was discovered by a member of the public on Station Street shortly before 3am on Sunday.
He was taken to hospital and was found to have suffered a ‘small’ bleed to the brain.
A statement from Hampshire Constabulary said: ‘It was reported to us that a man had been located shortly before 3am on the zebra crossing on Station Street, by Portsmouth and Southsea Railway Station, leading on to Commercial Road, with significant injuries to his face.
‘He was taken to hospital with two fractures to his face, bruising and swelling, and a small bleed to the brain.
‘He has since been discharged.’
Officers are working to establish the circumstances behind the injuries.
They are appealing to any potential witnesses for help.
Hampshire police said the man is believed to have travelled from Catherine House, on Stanhope Road, towards the railway station at approximately 2.30am.
They added that the man was found knocked out by a member of the public.
‘We would like to hear from anyone who may have seen a man who appeared to be injured between 2.30am and 3am or an altercation taking place’, Hampshire Constabulary added.
‘Perhaps you were in the area at the time and witnessed something suspicious?
‘Or maybe you were driving through the area and have Dash Cam footage that may assist our investigation?
‘Anyone with any information that may assist our enquiries is asked to contact us on 101 quoting reference 44220243448.’
A report can also be submitted online here.