The incident happened near to Wellingborough Road on Thursday, January 20, at about 12.40am, when a man was approached by a group of strangers and punched several times, knocking him to the floor.
One of the group then stole the iPhone 11 out of one of his pockets before they all walked off towards St Edmunds Road.
The main offender was described as a white man, about 6ft 2in, of a medium build, in his mid-20s, with short, dark-coloured hair, clean shaven and wearing dark clothing.
A second suspect is described as a mixed race man with medium length afro hair, about 5ft 8in, in his mid-late 20s, of a skinny build, and clean shaven.
Another suspect is described as a white man, about 6ft, of a medium build, in his mid-20s, clean-shaven, with short brown hair and wearing dark-coloured glasses.
Witnesses or anyone with information, particularly a man stood on Wellingborough Road at the time who may have seen the incident take place, should call Northamptonshire Police on 101.