Police received reports of an assault between 4am and 5am on Sunday.
The incident on Above Bar Street, near nightclub Café Parfait, resulted in a many suffering an injury to his head.
A 28-year-old man from Southampton was arrested on suspicion of causing actual bodily harm and has been released on police bail until September 15.
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Officers are now appealing for anyone with information to come forward to help trace the victim.
The man left the scene before police arrived.
A spokesperson from Hampshire Constabulary said: “We’re keen to speak to him to make sure he’s ok and to get further information about what happened.
“If this is you, or if you witnessed the incident, please contact us online or on 101 quoting 44250262105.”
