The 18-year-old was in a field when he was assaulted late at night.
Hampshire police have not released a description of the assailant.
It’s understood police yesterday cordoned off an area on the hill while the investigation continues.
A spokesman said: ‘We were called at 00.13am (on Tuesday, November 16) after an 18 year-old man reported that he was the victim of a serious sexual assault in a field on Portsdown Hill.
‘Specialist officers are providing support to the victim.
‘Enquiries are ongoing to establish what happened.’