Police are now appealing for witnesses following the incident in The Ridings in the early hours of yesterday (Sunday) morning.
The assault happened between 12.30am and 1am, near the Cordwainer pub, following an altercation between the victim and another man.
The offender is described as black, about 5ft 11, with a medium build, and in his mid-30s. He had short black hair and a trimmed beard.
Witnesses, or anyone with information, are asked to contact Northamptonshire Police on 101. Alternatively, they can call independent charity Crimestoppers on 0800 555111.