Nonconductive irrigation fluids used during transurethral resection (TUR) of the prostate can cause fluid overload and dilutional hyponatremia. TUR syndrome is generally defined as serum sodium at or below 125 mmol/L with cardiovascular and neurologic symptoms. The aim of this study was to evaluate the effects of Goreisan, a traditional Japanese Kampo medicine, on serum sodium levels and the occurrence of TUR syndrome in patients undergoing TUR of the prostate. This was a randomized-controlled trial. This trial was conducted at the Osaka Medical College Hospital and Keneikai Sanko Hospital. Fifty patients scheduled for TUR of the prostate were included. Patients in the Goreisan group (n = 23) received 2.5 g Goreisan orally on the night before surgery and on the morning of surgery. The control group (n = 27) did not receive Goreisan. Surgical procedures, perioperative management, and patient monitoring were otherwise the same in both groups. The primary outcome was occurrence of TUR syndrome. The secondary outcome was serum sodium level. Serum sodium remained above 125 mmol/L in all patients, so none of the patients met the criteria for TUR syndrome. However, the Goreisan group had significantly higher intraoperative sodium levels (p < 0.001) and significantly higher intraoperative (p = 0.008) and postoperative (p = 0.02) hemoglobin levels than the control group. These findings indicate that preoperative Goreisan administration can help maintain serum sodium levels in patients undergoing TUR of the prostate.