The 26-year-old victim was walking towards the St Stephen’s shopping centre with her mum and two children when she saw the man being bothered.
Police said the group seemed to be behaving in a threatening manner towards the man, who is believed to be of Indian origin.
When the woman intervened, she was assaulted by a girl aged around 15 years old.
The incident took place in Londesborough Street between 4.10pm and 4.30pm on Saturday, October 15.
Today investigating officers issued an appeal to trace a potentially crucial witness.
They want to speak to man who accompanied the woman to the shopping centre after she was assaulted.
The man, who is said to visit the centre regularly, spoke very little English and said he had only been in the country a short time.
He is described as being in his late 50s to early 60s and around 5ft 6in, with very short dark hair, which was greying. He wore a blue jacket and white baggy trousers.
Anyone who witnessed the incident, or who believes they may know the identity of the man, is asked to call Humberside Police on 101, quoting crime reference number 2222053.