She was near the Bridgewater Drive side of Abington Park when she was grabbed from behind and assaulted.
The incident happened between 7pm and 8.15pm on Sunday (May 27) and it is believed there would have been several people in the park at the time.
A Northamptonshire Police spokesperson said: "The suspect is described as white, about 6ft, in his late 30s, with short, mousey hair and a stubble beard.
"He was wearing black or navy blue trousers and a zipped khaki-coloured jacket."
Officers are asking anyone who witnessed the incident or saw anything suspicious to contact police.
Witnesses, or anyone with information, are asked to call Northamptonshire Police on 101, or Crimestoppers anonymously on 0800 555111.