The incident took place between 6.30pm and 8pm on Saturday evening, as the victim was walking with two friends in the underpass close to Crowthorne Road North, Downshire Way and Coningsby, in Bracknell.
The group were approached by a male, who performed a sexual act in front of them.
The girls ran away, but the male chased the group and touched the victim inappropriately.
The offender is described as a white male, between 5ft 6in and 5ft 7in tall and of average build.
He was wearing a red bandana across his face, a grey or blue tracksuit top, grey tracksuit bottoms and white trainers.
Thames Valley Police is appealing for information following the assault.
Anyone with information about the incident should contact Det Sgt Peter Wall on 101 or, to remain anonymous, call Crimestoppers on 0800 555 111.