The present study examined the efficacy and safety of yokukansan (YKS) in neuropsychiatric symptoms in patients with Parkinson's disease (PD) using the neuropsychiatric inventory (NPI). Twenty-five patients with PD (M:F 14:11; age 72 years) were enrolled and treated with YKS (7.5 g/day) for 12 weeks. The NPI was assessed at 0, 4, 8, 12 and 16 weeks. The patient's motor function and progression were evaluated using the Unified PD Rating Scale part III (UPDRS-III) and Hoehn and Yahr scale, respectively. The serum potassium concentration (sK) and all adverse events were recorded. The median NPI total score significantly decreased from 12 points at baseline to 4.0 points at 12 weeks (p = 0.00003). Within each NPI subscale, significant improvements were observed in hallucinations, anxiety and apathy. These symptoms tended to worsen after the completion of YKS treatment. Delusions, agitation, depression, euphoria, disinhibition, aberrant motor activity tended to improve but irritability showed no change. The median NPI subtotal scores, positive symptoms (delusions-hallucinations-irritability) significantly decreased (p = 0.01660) and negative symptoms (anxiety-apathy) significantly decreased (p = 0.00391). Both UPDRS-III and the Hoehn and Yahr scale showed no significant change. sK decreased mildly from 4.26 ± 0.30 to 4.08 ± 0.33 mEq/L. Two patients showed hypokalemia lower than 3.5 mEq/L without any corresponding symptoms; two patients showed listlessness and one patient showed drug eruption. Each recovered after discontinuation of YKS. YKS improved neuropsychiatric symptoms associated with PD, including hallucinations, anxiety and apathy without severe adverse events and worsening of Parkinsonism.