Haddad, 16, was found stabbed to death in early July outside a block of flats in Södertälje, 40 kilometres south west of Stockholm.
The girl’s killer has a history of mental illness.
Södertälje District Court said his crime would have resulted in a life sentence had he not been found to have suffered from paranoid schizophrenia at the time of the murder.
He was not acquainted with his victim. After the murder, the 39-year-old contacted a psychiatric ward in Södertälje where he had previously been a patient to inform staff there of his crime.
Haddad was found dead with multiple stab wounds in the residential area of Rosenlund.
The 39-year-old was arrested near the crime scene following a tip off from a witness.
Norba Haddad lived in another part of Södertälje and was visiting the Rosenlund residential area. She was outside when she was attacked and stabbed repeatedly.