Background: Preoperative anxiety and postoperative delirium affect both short- and long-term prognoses in patients with cancer; therefore, these conditions require early prevention and treatment. However, no standard preventive or therapeutic methods have been established for them. Yokukansan, a Japanese herbal medicine for the treatment of insomnia and anxiety, causes relatively few adverse drug reactions and effectively improves the behavioral and psychological symptoms of dementia. Thus, it is expected to be useful for treating and/or preventing perioperative psychiatric symptoms in patients with cancer. The objective of this study is to clarify the therapeutic effect of Yokukansan for preoperative anxiety and its preventive effect on postoperative delirium in cancer patients, as well as to confirm its safety profile. Methods: This study is a randomized, double-blind, placebo-controlled study in cancer patients scheduled to undergo tumor resection. Patients who provide consent are randomly allocated to receive oral administration of Yokukansan or placebo, and study drug administration is continued for 4 days or longer prior to surgery. We defined two primary endpoints, change in preoperative anxiety and incidence of postoperative delirium. Secondary endpoints are severity score of postoperative delirium, duration of postoperative delirium, amount of benzodiazepines used prior to surgery, amount of antipsychotic agents used after surgery, and number of postoperative hospitalization days. We plan to complete the analysis on March 31, 2021. The target number of registered patients is 110 per group, or 220 in total. Discussion: This study is the first randomized, double-blind, placebo-controlled study intended to clarify the effects of a Japanese herbal medicine, Yokukansan, in the prevention and treatment of perioperative psychiatric symptoms in patients with cancer. The trial was initiated on August 14, 2017, with 195 subjects randomized by October 5, 2018. Trial registration: UMIN Clinical Trials Registry (UMIN-CTR), UMIN000027561. Registered on 31 May 2017.