It occurred on the steps near Skeldergate Bridge just after 1.30am on Monday 3 March 2014.
The victim, a local woman aged in her early 20s, was walking home alone after a night out with friends when she was subjected to a serious sexual assault by an unknown man who she had walked with for a short distance.
She is receiving support from specially trained police officers.
The suspect is described as white, aged in his 20s, and was possibly wearing a tracksuit.
Police enquiries remain ongoing to locate the man, and the scene of the incident is cordoned off to allow for forensic examinations and searches.
Patrols have also been stepped up in the area to offer reassurance to the local community while the investigation continues.
Residents are urged to come forward with any information that could assist the police.
Detective Inspector Andrea Kell, of York CID, said: If you were in the Skeldergate Bridge area of York at around 1.30am on Monday and possibly recall seeing a man and woman, I urge you to come forward immediately as you may have vital information which could help the investigation.
I am particularly keen to speak to anyone who may have seen a woman trying to flag down vehicles on Skeldergate Bridge shortly after the incident. Again, please do not delay in contacting the police.
If you can help, please call North Yorkshire Police on 101, select option 2, and ask for York CID. Alternatively, information can be emailed to firstname.lastname@example.org
Or call Crimestoppers anonymously on 0800 5551111, quoting reference number 12140034199.