The tag will alert police if the girl - who cannot be named for legal reasons - leaves the estate where she lives.
Magistrates in Walsall decided to make her wear the tag after she broke a court order that banned her from going into the town centre.
She was originally ordered to stay out of the town centre after she was accused of mugging another 12-year-old girl for her mobile phone and £5 cash.
The tag was fitted when she broke the bail conditions.
In the past when kids have broken the law, the police have sometimes been forced to let them go and some commit even more crimes.
It is hoped tagging will help stop this.