Sanita Muceniece, from Latvia, was last seen on October 21 at 11.30pm and has previously lived in the Croydon area.
Sergeant Ross Wickings said: “Sanita has been missing before and is known to visit the Croydon area. She particularly likes to use the subway. We are concerned for her wellbeing because of her age and urge her to get in touch to let us know she is okay.
“Sanita is slim, about 5’ 11” tall, with long straight blond hair and blue eyes. She wears heavy gold necklaces and gold hoop earrings.
“I would appeal to anyone who recognises her or knows of her whereabouts to contact Sussex Police on 101 or the independent charity Crimestoppers on 0800 555 111.”