The teenage Fulwood High School pupil was followed as she walked home from a friend's house on Black Bull Lane.
The man, who neighbours had reported hanging around the area earlier in the week, is reported to have followed 13-year-old Nicole and grabbed her bottom.
Mum Joanne Dickinson said: "He grabbed her bum and put his arm around her waist.
"She was frightened and yelled and fortunately one of the neighbours came out and yelled at him and he ran off.
"She's only 13, she was terrified.
"We only moved to the area recently and part of the reason for the move was so that Nicole could walk to school and go and see her friends, but now she's afraid to go out.
"Fortunately we have good neighbours who were looking out for her.
"There's been reports of a man hanging around in the area for some days now, and I believe that some people have given photos and videos to the police."
The incident happened at around 4.30pm on Wednesday on Black Bull Lane in Fulwood.
Lancashire Police have been informed.