The robbery took place at around 9.30pm as the 20-year-old victim was walking in Cheyne Way and about to cross the bridge, police said.
He was then approached from behind by a man who stole his phone. The offender then ran off into an alleyway that leads to Cheyne Way.
The man police want to speak to is described as being white, around 5 feet 10 inches tall with a bald head. He was wearing dark rectangular glasses and spoke with a local accent.
Constable Daniel Lewis said: “I am appealing for anyone with information about this incident to come forward and speak to us.
"It is possible that someone saw or heard what happened, or saw the offender in the area. I would urge them to get in touch.”
Anyone with information should contact Hampshire Constabulary on 101 quoting reference 44170092395.