In March 2009, Calwyn Fearon, 33, told police he killed his 52-year-old girlfriend because she was the leader of a group of ghosts that had been tormenting him, police said.
On Tuesday, a panel of judges found Fearon not guilty by reason of mental defect.
Prosecutors argued that Fearon was aware of the wrongfulness of his actions, but a forensic psychiatrist testified that Fearon suffered from chronic paranoid schizophrenia.
Fearon will remain in custody while he undergoes another psychological evaluation.
He's expected to be committed to the custody of the Connecticut Psychiatric Security Review Board.