The incident happened between St James Mill Road and St James Retail Park in Northampton at around 9pm on Tuesday, November 8.
The woman was assaulted by a man, who is described as black, 50-years-old and 5ft 8in tall with a slim build.
Northamptonshire police are appealing for a passer-by who came to the victim’s aid to contact police, along with anybody else who was in the area at the time.
Anyone with information is asked to contact Northamptonshire Police on 101. Alternatively, they can call independent charity Crimestoppers anonymously on 0800 555111.