Aaliyah Chen, 15, was last seen in Sidcup at around 9.30pm on the evening of Sunday, 5 September.
She was reported missing the following morning.
Officers believe that Aaliyah may be in the company of an older man and a high risk missing person investigation is under way to find her.
The pair may have travelled away from Sidcup although it is thought that they are likely to still be in London.
Anyone who sees Aaliyah should call 999 immediately, providing the reference CAD 4086/08Sep.
Anyone with other information about her whereabouts should call 101.