Tilli disappeared from Smethwick, West Midlands, at around 6.40pm on Tuesday evening and has not been seen since.
The schoolgirl was described as wearing jeans and a beige top, said Birmingham Police.
Tilli was also carrying a pink and black backpack, said the force.
COPS have launched an urgent hunt for an 11-year-old girl after she went missing.
Officers did not give any information regarding her physical appearance.
