Extrapyramidal symptoms (EPS) are common adverse effects of antipsychotic treatment. This study examined the effects of the traditional Japanese herbal medicine (kampo) shakuyaku-kanzo-to on EPS during antipsychotic treatment. Twenty-two Japanese patients with psychiatric disorders who had developed EPS during antipsychotic treatment were randomly allocated to receive either shakuyaku-kanzo-to (7.5 g/d) or biperiden (3 mg/d) for 2 weeks. Extrapyramidal symptoms were evaluated using the Drug-Induced Extrapyramidal Symptom Scale (DIEPSS) and the Barnes Akathisia Rating Scale. Plasma levels of the monoamine metabolite homovanillic acid and serum prolactin levels were measured to investigate the mechanisms of action of shakuyaku-kanzo-to. Twenty of the 22 patients completed the study (10 patients in the shakuyaku-kanzo-to group and 10 patients in the biperiden group). There was a time effect on the Drug-Induced Extrapyramidal Symptom Scale total score (P < 0.01), suggesting that both shakuyaku-kanzo-to and biperiden decreased EPS. Notably, there was a time × drug interaction in dystonia, suggesting that shakuyaku-kanzo-to had a greater effect on dystonia compared with biperiden. No significant changes were observed in plasma homovanillic acid or serum prolactin levels after 2 weeks of treatment in either group. The effects of shakuyaku-kanzo-to on abnormal muscle tonus and dopamine D2 receptors may have contributed to improve EPS. These results suggest that shakuyaku-kanzo-to may be useful in decreasing EPS, especially dystonia, in patients undergoing treatment with antipsychotic agents.