Police on patrol were informed of the incident in the Bournemouth Pier area at around 7.45pm on Friday 19 July.
Dorset Police say the girl was touched inappropriately by a man.
A 20-year-old man from London was arrested on suspicion of sexual assault and has since been released on police bail while further enquiries are carried out.
A Dorset Police spokesman said: “Officers, who received the report directly whilst on foot patrol in the immediate area, responded and it was reported that a man had sexually assaulted a teenage girl by touching her inappropriately.”
Approaching the quay
The police spokesman added: “Officers were in the area as part of their enhanced patrols in Bournemouth town centre and the beach area and we will continue to work with our partners to ensure a visible presence in the surrounding area.
“The police will be in contact with the public and can be contacted by anyone with information or concerns.”
This incident is believed to be separate from the large fight that was reported at Pier Approach and to which police were called just 15 minutes later.
