The victim, who is in his 20s, suffered a minor head injury and cuts and bruises in the attack, which happened as he walked along the towpath near Victoria Street and Severn Street in Castlefields and the English Bridge at about 11.50pm.
The group of up to five offenders approached the victim from behind and, following the robbery, ran back in the direction of the weir.
The victim got to his feet and realised his mobile phone had been stolen, before coming across another member of the public, who called the police.
An ambulance was called and a search carried out in the area by police but the offenders were not found.
Police Constable Naomi Teckoe, who is investigating the robbery, appealed for anyone who might have seen what happened or a group of males in the area during the relevant time to contact police on 101.