A man was assaulted while drinking in the Ock n Dough pub in Farm Road, Wellingborough, on Friday evening (July 21).
Sometime between 10.30pm and 11pm, the man tried to intervene in an argument between two other men and he was punched in the face by a woman.
The woman is described as white, about 5ft 10in, with a medium build and aged 20 to 25.
She had brown hair, slightly longer than shoulder length.
Witnesses, or anyone with information about the assault, is asked to contact Northamptonshire Police on 101 or Crimestoppers anonymously on 0800 555111.