The 28-year-old victim was attacked by a man in the Rawcliffe area of the city at about 1am today.
Police said the woman believes she was followed from Malton Way into Rawcliffe Lane before the assault took place on Rawcliffe Drive.
Her attacker is said to have been wearing a grey hooded top with the hood up.
Officers were today trawling through CCTV footage and carrying out other enquiries in the area.
A spokesman for North Yorkshire Police said: “The victim is being supported by specially trained officers while the investigation continues.
“Reassurance patrols have also been stepped up in the area to address any community concerns.”
Police say they want to hear from witnesses to the incident or anyone who saw a man in a grey hooded top in the area around the time that the attack happened.
Officers also want to speak to two people – a woman and a man who was walking his dog – who went to the aid of the victim.
Anyone with information is asked to ring North Yorkshire Police on 101 or Crimestoppers anonymously on 0800 555111.