Sussex Police officers are appealing for information about a disturbance at premises in Hastings town centre.
The incident happened inside and then outside the Yates Wine Bar in Cambridge Road at about 2.45am on December 18.
A 35-year-old man was later arrested by officers on suspicion of sexual assault on a woman, possession of a class A drug, racially aggravated harassment, and drunk and disorderly behaviour.
He remains on bail while the investigation continues.
Police are seeking to trace witnesses to the incident, and wish to identify the victim of the sexual assault.
Witnesses or anyone with information is asked to report it to Sussex Police online or call 101 and quote serial 138 of 18/12.