The 27-year-old victim was walking up a staircase away from a Victoria line platform at Highbury and Islington station when she was approached by a man and groped, police say.
An investigation was launched after the woman reported the encounter shortly before 7pm on November 2.
On Friday, police released a CCTV image of a man they want to track down in connection with the assault.
Investigating officer PC George James, said: “This was a brazen assault that happened on a busy staircase.
“We will simply never tolerate unwanted sexual behaviour and we are working hard to trace the person responsible.
“I am keen to speak with the man in the CCTV images as I believe he has vital information that could help our enquiries.”
Anyone with any information can contact BTP by sending a text to 61016 or by calling 0800 40 50 40.
