The victim said she felt a hand go over her mouth and was pulled backwards in Scotland Road, Rochdale, this week and was then dragged off the street.
The attack happened at about 6.45AM on Tuesday, whilst the elderly woman was making her way to a local shop.
A spokesperson for Greater Manchester police said that the woman lost consciousness and woke up on the floor having been raped. She then made her way to a nearby butcher's shop and told staff of the assault.
Supertintendant Alistair Mallen said that it was a 'vile, wicked and despicable crime.'
Police are now appealing to the wider Manchester community to get in touch if they know anything of the incident.
'I am sure the entire community will share our revulsion at this,' Mallen said.
'I now want people…to imagine if that had been your mum or grandma.'
'I want to harness these feelings and use them to help the police do their job - catch this man and put him before the courts.'
At the time of the attack the 90-year-old was wearing a white floral dress and a cardigan. Her attacker was described as white.
Mallen added that the woman is now being supported by police officers.
'We are doing everything possible to reassure her that we will catch the man responsible.'
Anyone with information should call police in Rochdale on 0161 856 8470.