The woman was walking home after finishing work at around 2.30am in the early hours of Sunday 26 October 2014.
As she was walked along Fossgate and into Walmgate, the man began walking with her and trying to talk to her.
He then followed her to her home where he entered the communal hallway with her before grabbing hold of her.
The woman then screamed and the man ran off along Walmgate in the direction of the city walls.
The man is described as white, aged between 20 and 25 years old, tall and skinny with short, dark brown or black hair. He was wearing a light coloured T-shirt and jeans.
Officers are appealing to anyone who witnessed the man walking with the woman, running off from the scene, or can help identify him, to come forward.
Detective Inspector Alan Rowan of York CID, said: Thankfully, the victim sustained only minor injuries during the incident, however she was left very shaken up by what happened. We are carrying out extensive enquiries to find the person responsible and information from members of the public could be key to helping us find the suspect.
The area was busy with people enjoying a night out at the time and I am appealing to these people and anyone else who was in the Fossgate and Walmgate area at around 2.30am, to contact the police as they could have important information. If you can help, please contact us without delay.
If you wish to remain anonymous, you can pass information to Crimestoppers on 0800 555 111.
Please quote incident number 12140184683 when passing on information.