We aimed to evaluate the radioprotective effect of hydrolyzed rice bran (HRB) on acute gastroenteritis due to chemoradiotherapy for treatment of cervical cancer. This placebo-controlled, double-blind study was conducted as an exploratory investigation of the colitis-inhibiting effects of HRB in alleviating acute-phase gastrointestinal side effects of chemoradiotherapy. The study involved 20 patients (10 in the HRB group, 10 in the control group). The patients in the control group underwent the same chemoradiotherapy regimen as those in the HRB group, but they received a placebo instead of HRB. The diarrheal side effect assessment score was lower in the HRB than control group, and a trend toward a reduction in diarrhea symptoms was observed with the oral intake of HRB. Additionally, no significant difference was observed in the administration of intestinal regulators and antidiarrheal agents, but again the assessment score was lower in the HRB than control group, and diarrhea symptoms were alleviated with the oral intake of HRB. A trend toward no need for strong antidiarrheal agents was seen. Although this study was an exploratory clinical trial, the results suggest that HRB may relieve diarrhea, an acute-phase gastrointestinal side effect of chemoradiotherapy.