The 12-year-old was about to get off the number 50 bus on Alcester Road, Druids Heath at around 4pm on Thursday 26 September when the woman grabbed her Samsung phone and ran off.
The suspect is described as black, around 5ft 2ins tall and was of slim build. She was wearing a brown coloured cardigan and a cream knee length skirt.
Detective Constable Lee Reeday, from Birmingham CID, said: “This was an opportunistic theft where the suspect saw an opportunity to grab the phone and took it.
“We know there were other people on the bus at the time it happened and would ask any witnesses or anyone who recognises the woman in the image to come forward.
“Everyone who uses the bus network in and around Birmingham is reminded to keep their valuables secure at all times and report any suspicious activity to police as soon as possible.”
Anyone with information should contact police on 101 or call the independent charity Crimestoppers, anonymously, on 0800 555 111.