Nazia Nazish, who has been missing for eight days, was last seen in the Deptford area of London on Tuesday, November 27.
Police officers in Hertfordshire believe the youngster, who is black, 5ft 5in tall and has long brown hair, could still be in Deptford.
When she was last seen she was wearing a black padded jacket and black skinny jeans.
PC Stuart Barnett said: “We are appealing to anyone who believes they may have seen Nazia, or know where she is, to get in touch with Hertfordshire police on the non-emergency number 101.
“And to Nazia, if you are reading this, please get in touch with us as soon as possible.”