Ellie Williams, 16, was last seen on Waddington Way, in Kingsway, Gloucester, at around 7pm on Monday night.
It is believed she may have travelled to the Smethwick area – and officers from Gloucestershire Police are now liaising with colleagues at West Midlands Police in a bid to trace her.
When she was last seen, Ellie was wearing black leggings, a black crop top, dark blue Nike jacket, light pink handbag and pink trainers.
A spokesman for Gloucestershire Police said: "Gloucestershire Constabulary is appealing for information that may help to locate a teenage girl who has been reported missing from Gloucester.
"Sixteen-year-old Ellie Williams was last seen on Waddington Way in Kingsway at 7pm on Monday night and has not returned since which has caused concerns for her welfare.
"She is described as being 5ft 10ins tall, of a heavy build, wears glasses and has very long red hair.
"When last seen she was wearing black leggings, a black crop top, dark blue Nike jacket, light pink handbag and pink trainers.
"It is believed that Ellie may have travelled to the Smethwick area and officers are liaising with colleagues at West Midlands Police."
Anyone who has seen Ellie, or knows of her whereabouts, is asked to call Gloucestershire Police or West Midlands Police on 101, quoting incident number 465 of May 17. If seen at the time of calling, call 999.
Alternatively information can be given to the Missing People Charity by calling 116 000 or online via www.missingpeople.org.uk.