The incident happened between 1am and 3am on Saturday, September 24.
A spokesman for Northamptonshire Police said: "The victim had left the town centre and was walking home. She passed the Salvation Army centre, onto St Andrews Street when the offender punched her and assaulted her.
"The suspect is described as white, around 5ft 10ins tall, of medium build, in his early 30s and with pale skin.
"He had short blonde hair and was wearing a white polo top and grey shorts," he added.
Anyone with information can contact Northamptonshire police on 101. Alternatively, they can call the independent charity Crimestoppers anonymously on 0800 555 111.