The incident happened in the early hours of Monday, June 30 last, but police have just released details.
The woman, who is in her 20s, was approached at William Street in Derry by a man who started to talk to her.
As she moved away, the man grabbed her around the upper part of her body and also indecently assaulted her.
She broke free and left in a friend’s car.
The man is described as being in his mid 20s, with dark hair and was of a thin build.
Information can be given to police at Strand Road on the non-emergency number 101.
Alternatively, it can be provided to the Crimestoppers charity anonymously on 0800 555 111.