A woman was sexually assaulted while talking on her phone at the bottom of Carrington Street in Kettering.
The victim was stood talking on her phone when a man walked past her and touched her inappropriately sometime between 1.10am and 1.25am on New Year’s Day.
The offender then walked up the street towards Bar Seven.
A police spokesman said: “The victim followed the offender who was stood outside the bar with two or three other men and challenged him.
“The offender and the other men left the area before police arrived.”
The offender is described as white, about 19-years-old, between 5ft 9in and 5ft 11in with a slim build and short mousy brown hair.
He wore a dark jumper with a white shirt.
Anyone with any information or who witnessed the group outside the bar can call Northamptonshire Police on 101 or they can call the independent charity Crimestoppers anonymously on 0800 555111.