Germantown Police Chief Peter Hoell posted a note on his department's Facebook page Saturday, asking if anyone knew where the ring came from.
He says the diamonds are real. Police think someone intentionally placed the ring on a branch of the tree in the police department's lobby.
For the past five weeks, the department invited people to bring their children to decorate the tree and get a stuffed animal in return. During that time, someone put the ring on the tree but left no explanation.
Germantown is about 15 miles northwest of Milwaukee.
The Milwaukee Journal Sentinel reports that anyone claiming the ring will have to provide a detailed description.