Edward Little, 15, from Brighton, was spotted in Charlotte Street, Bath at 5pm on Friday, November 11.
Police described him as about 5ft 5ins tall and of slight build, with black hair and brown eyes. He is of dual heritage and speaks with a London accent.
Until last week Edward had been in contact with relatives through social media, but his family have now not heard from him since Sunday, November 20.
A police spokesman said: “Edward’s family live in the Brighton area of Sussex and it’s not clear why he was visiting Bath. He’s also thought to have connections in Wiltshire and the Thames Valley areas.”
Anyone with information which could help officers find Edward and make sure he is safe and well is asked to get in touch through www.avonandsomerset.police.uk/contact, or by calling 101, quoting reference 5216251874.