The 26-year-old victim was attacked at the door of the Life O'Reilly pub in Dixon Avenue in the Govanhill area of Glasgow at about 11.45pm on Sunday.
Police said the woman's friends had walked ahead into the pub when a man came up behind her and touched her inappropriately.
He made off along Dixon Avenue towards Langside Road and Allison Street after the woman's shouts for help attracted her friends and two men who were in the pub.
She did not need hospital treatment but was said to be "very shaken".
The suspect is described as Eastern European, in his mid 20s and about 5ft 7ins with dark spiky hair and a strong build. At the time he was wearing a light top, slim jeans and trainers.
Detective Constable Murray Rolley, from London Road CID, said: "Officers are carrying out extensive inquiries including checking CCTV and going door to door in an effort to trace the man responsible.
"I am particularly keen to speak to the two men who helped the woman outside the Life O'Reilly pub as they may have information which can help progress the inquiry.
"Both men are described as being in his 50s, one was tall with a slim build and wearing a beanie style hat and the other may have been a little older, in his late 50s and may have had a dog with him."
Anyone with information is asked to contact Police Scotland on 101.