The flashing happened on Monday, April 29, between 1.45pm and 2.15pm between the second and third roundabouts on Long Croft Road.
The woman was walking along the road when a man next to a small white van called out to her.
He indecently exposed himself to the woman, who ran off.
A Northamptonshire Police spokeswoman said: "The suspect is described as a white man, aged between 25 and 35, of average build, with short mousey brown hair.
"He had a distinctive tattoo on the side of his neck and was wearing a blue t-shirt and dark blue cargo-style trousers.
"He drove a small white van with red writing on the side."
Witnesses or anyone with information regarding this incident should contact Northamptonshire Police on 101, or Crimestoppers anonymously on 0800 555111.