In recent years, herbal therapies are gaining popularity as alternatives to prescribed drugs. St. John’s Wort (SJW), which is also named as hypericum perforatum, is one of the most frequently used herbal agents in the treatment of psychiatric disorders. SJW has been shown to be efficacious in treating mild to moderate depression, insomnia, and anxiety disorders. Although exact mechanism of action is not completely understood, its active components are suggested to have antidepressant properties. Despite its beneficial therapeutic effects, SJW can also cause unexpected side effects. Herein, we present a case of a 47-year-old female who developed a psychotic episode after taking St. John’s wort extract. The patient had been previously well, with no previous medical or psychiatric history. Following a one-week period of disorganized speech, bizarre behavior, delusions of reference, visual and auditory hallucinations, social/emotional withdrawal, and self-neglect, she was admitted to psychiatry outpatient clinic. Neurological examination, electroencephalogram (EEG) and cranial magnetic resonance imaging (MRI) were normal. The symptoms were interpreted as a psychotic episode, associated with SJW. Therefore, SJW was stopped and intramuscular haloperidol, 10 mg/day, was started. By the seventh day of treatment, a significant improvement in his symptoms was observed and intramuscular haloperidol was changed to oral olanzapine, 5 mg/day. The 3-month follow-up revealed no psychotic symptoms. This case suggests that sudden onset psychotic disturbances inpatients in the absence of other known organic factors could be related to treatment with St. John’s wort, and that cessation of this can markedly improve psychiatric morbidity. Furthermore, treatment with antipsychotic medication can lead to alleviation of St. John’s wort associated psychotic symptoms