Millie, whose surname Sussex Police did not provide, has been deemed to be missing from Worthing.
Police described her as 5 foot 7 inches tall and said she wears thick black rimmed glasses.
COPS are urgently hunting for a 14-year-old girl who went missing and "may be with a 20-year-old man".
Millie may be with a 20-year-old man who is known to her, according to Sussex Police.
Cops said that the schoolgirl may have travelled to the Croydon area.
