The incident happened between 8pm and 9pm on Tuesday, April 4, near the junction with Wellington Street, outside Betfred and Papa Johns Pizza.
The victim was punched from behind and dropped his mobile phone.
A man then picked up the phone and two men ran off towards Wellington Street.
The men were both white, about 5ft 10in, aged 25 to 30.
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One of the men was wearing a black puffa-style coat and the other was wearing a cream-coloured jacket.
Witnesses, or anyone with information, are asked to contact Northamptonshire Police on 101.
Alternatively, they can call independent charity Crimestoppers on 0800 555111.