Thames Valley Police (TVP) said that at approximately 8.10am on Monday (September 21), the girl was walking on The Redway at the back of Rochfords, Coffee Hall.
Officers said she walked towards two men, one of whom grabbed her arm and pulled her towards him before touching her inappropriately over her clothing.
A second man, who was with the offender, was heard to say that the offender should not have done what he did, say police.
Both men then walked off in the direction from The Redway towards Rochfords, Coffee Hall.
TVP said the offender is described as a slim white man, aged in his late 30s, approximately 5ft 9ins tall, with brown curly hair, slight stubble, and he spoke with what is believed to be a Polish accent.
The man who was with the offender is described by police as white, slim, aged in his late 20s to early 30s, around 5ft 9ins to 6ft tall, with greying curly hair and no facial hair. This person, according to police, was not involved in the offence.
Investigating officer, detective inspector James Horseman, said: “I need the public’s help to identify both the offender and the witness who was with the offender.
“Although this was an isolated incident, it was extremely distressing for the victim and she is being supported by officers.
“We have had officers in the area in an increased presence for high visibility reassurance patrols.
“I would appeal to anybody who witnessed this incident to please get in touch with us by calling the 24-hour non-emergency number 101, quoting reference number 43200297047.
“Alternatively, you can make a report online, or for 100% anonymity, call the independent charity Crimestoppers on 0800 555 111.
“I would particularly appeal to the man who was with the suspect. It is apparent that you know this behaviour was not acceptable, and so please pick up the phone and speak to officers to give us information that can help this investigation.
“Any information given will be treated in the strictest confidence.”