The German-Moroccan teenager, identified only as Safia S., was charged on August 12th with attempted murder, grievous bodily harm and support for a foreign terrorist organisation, the federal prosecutor's office said in a statement.
A second suspect, named as 19-year-old German-Syrian national Mohamad Hasan K., was charged on suspicion of failing to report Safia S.'s planned attack.
The girl, then 15, stabbed the federal officer during a routine check at Hanover train station in the north of the country, before being overpowered by another police officer.
She has since been in preventive custody.
Prosecutors said they believe the girl was radicalised by November 2015 at the latest.
In January, she travelled to Istanbul where she was due to meet two Isis middlemen who were meant to help her cross into Syria.
The plan was aborted when her mother brought her back to Germany, but the Isis members ordered her to carry out an "act of martyrdom" when she arrived home.
She told Mohamad K., who was already aware of her trip to Istanbul, about her plans but he did not go to police, prosecutors said.
The officer suffered a life-threatening injury in the stabbing but survived after surgery.
Germany endured four attacks within a week in July, including two assaults claimed by Isis, stoking a security debate focused on potential risks posed by the arrival in 2015 of around one million asylum seekers.
The axe rampage, a shooting spree, a knife attack and a suicide bombing left 13 people dead, including three assailants, and dozens wounded.