15-year-old Anistasia (Annie) Lovelock is missing from Llanwrthwl, near the Llandrindod Wells area.
She was last seen around 3.30pm yesterday (November 19).
Dyfed-Powys Police said Annie has links to the Buckinghamshire area and may be heading that way.
Police said she may also have travelled to the Carmarthen area.
A search of the local area has taken place.
Annie has been described as 5ft 6inches tall, with a slim build and mousy brown short hair.
She was wearing a dark blue hoody with navy tracksuit bottoms and red/pink Nike trainers.
Dyfed-Powys Police are urging anyone who sees Annie or is in contact with her and knows where she is to get in touch by calling 101.
If you are deaf, hard of hearing or speech impaired you can text the non-emergency number on 07811 311 908, quoting Ref: 251 of Nov 19.