The incident happened at around 1.15am on Wednesday 22 August 2018 on Grammar School Lane at the junction of Hutchinson Drive.
A woman in her fifties was walking in company with a man when he sexually assaulted her by touching her.
Anyone who witnessed the incident or has any information is asked to contact police. Dial 101, press 2 and ask to speak to the Northallerton Serious Crime Team.
Please quote reference number 12180159193 when passing on any information.