The assault happened on Saturday, January 30, between 12.30am and 3am, in NBs, Bridge Street, Northampton.
The victim was in the bar when she was confronted by a man who would not let her pass. He then grabbed and touched her inappropriately and kissed her on the neck. He was with three other men who were laughing and encouraging him.
The woman managed to attract the attention of another man, who pushed the offender and helped the woman get away from him.
The offender is described as white, about 6ft, with a medium build and in his early 30s. He had mousey blonde, short, spikey hair and spoke with an Eastern European accent. He was wearing a cream jumper.
Police are urging anyone who may have witnessed the incident to contact them. They would particularly like to speak to the man who helped the woman, who was black, about 5ft 10in and in his 20s.
Witnesses, or anyone with information, can call Northamptonshire Police on 101. Alternatively, they can call independent charity Crimestoppers anonymously on 0800 555111.