The woman was sat on a bench on Montpellier Hill in Harrogate, across from the Blues Bar.
Between 9pm and 10pm on Saturday, 13 August 2016, a man sexually assaulted her by touching her, before exposing himself to her.
A man in his fifties has been arrested in connection with the incident and remains in police custody at this time. But it is believed that a number of people were in the area at the time and some of them were concerned about the woman.
Officers are appealing to them to come forward along with anyone else who witnessed the incident.
If you wish to remain anonymous, you can pass information to Crimestoppers on 0800 555 111.
Please quote reference number 12160149144 when passing on information.