The 19-year-old woman was pushed into an alley and subjected to a terrifying attack in Lings by a man who police believe is responsible for at least three other assaults in the same area of town.
Her boyfriend's father, who lives in Lings, said: "It's disgusting.
"Fortunately the guy got disturbed by a passing car and he ran off, so it could have been worse.
"But she's still shaken up by it.
"There are a lot of dodgy characters in this part of town and if I was a woman, I would be terrified to go out on my own after dark."
The attack took place at about 7pm on Monday, February 18, in Lings Way and detectives have described the suspect as black, in his 30s, about 6ft and of medium to muscular build.
A man matching the same description is also wanted in connection with three other attacks, including one on a 14-year-old girl in the Goldings area on the evening of Wednesday, February 27.
Another 14-year-old was assaulted near the McDonalds restaurant in Weston Favell in December, before a woman was attacked in Billing Brook Road, Lings, on January 31.
And police are still looking for another man who sexually assaulted a primary school-aged girl in Wade Meadow Court, Lings, on Friday, February 8.
People living in the area say they are disturbed, but not surprised, to hear of the attacks.
Emma Newton, of Great Holme Court in neighbouring Thorplands, said: "I have lived in the Eastern District all my life and it is a dodgy area, so this doesn't surprise me at all to be honest.
"I've wanted to move away for some time and this kind of thing makes me even more keen to leave."
Tina Holden, of Water Pump Court, Thorplands, said: "It's a bit worrying because it is all so close to home. Because of the type of area I live in, I don't like to go out on my own after dark."
Anyone with information about any of the attacks can call Northamptonshire Police on 08453 700700 or Crimestoppers, anonymously, on 0800 555 111.
ed.omara@northantsnews.co.uk