Mia Walker is believed to have last been in contact with family at around 7pm on Sunday 11 December 2022. She was last seen in Wesley Road during the morning of Friday 9 December 2022.
She is described as approximately five feet seven inches tall with long red hair that is brown at the roots.
Inspector Jon Wasey, of Dorset Police, said: “We are concerned for the welfare of Mia due to the passage of time since she was last seen.
“I would urge anyone who has seen Mia, or a girl matching the description given, to please come forward.
“It is believed that Mia is not alone and we would ask anyone who has seen her either on her own or in the company of another girl to contact Dorset Police.”
Anyone with information or knowledge as to Mia’s whereabouts is asked to contact Dorset Police at www.dorset.police.uk/contact or by calling 101, quoting occurrence number 55220199962.