The woman, who did not want to be named, said her friend was walking along Portland Road shortly before 11pm on Sunday (November 7) when she was approached.
The man is alleged to have exposed himself at her and then masturbated towards her.
The victim was on one side of the road, near the former Sandicliffe garage, while the perpetrator was near a bus stop on the opposite side.
The Dispatch has been told the incident has been reported to the police and the woman has urged people to be on guard and vigilant - and report any further incidents like this to the police.
The Dispatch has approached Nottinghamshire Police for comment.