Background: Stomatitis, a frequent adverse event in patients undergoing chemotherapy, can lead to pain, ulcers, bleeding, and malnutrition due to reduced oral intake, which not only deteriorates quality of life but may also result in dose reduction or interruption of chemotherapy. However, there is currently no standard approach for preventing chemotherapy-induced stomatitis. Therefore, this study aimed to assess the efficacy and safety of a dexamethasone-based mouthwash for preventing chemotherapy-induced stomatitis in patients with early breast cancer. Methods: We conducted a multicenter, randomized, controlled phase II trial. Patients with early-stage breast cancer scheduled for epirubicin and cyclophosphamide (EC) or docetaxel and cyclophosphamide (TC) therapies were allocated in a 1:1 ratio to the intervention and control groups. The sample size was set to 120 cases based on the following assumptions: stomatitis incidence of 50% in the control group and 30% in the intervention group, alpha error of 0.2, one-sided power of 0.9, and dropout proportion of 10%. The intervention group received chemotherapy, oral care, and a dexamethasone-based mouthwash (10 mL, 0.1 mg/mL; swish for 2 min and spit, 4 times daily for 9 weeks), and the control group received chemotherapy and oral care. The primary endpoint was the incidence of stomatitis that was compared between the two groups. Results: There were 60 patients in the control group and 61 in the intervention group. One patient in the control group and two in the study group withdrew consent; one patient in the control group was excluded because of discontinuation of the first cycle of chemotherapy and unavailability of data related to the incidence of stomatitis. Finally, the data of 58 patients in the control group and 59 in the intervention group were analyzed. There were no differences in age and chemotherapy regimens between the groups. The incidence of stomatitis was 55% in the control group and 38% in the intervention group (risk ratio 0.68; 80% confidence interval, 0.52 – 0.88; p = 0.052). The grade of stomatitis (0/1/2/3) was significantly lower in the study group (intervention group vs. control group: 37/13/7/2 vs. 26/16/7/9, p = 0.03). There were no differences in the duration of stomatitis between the groups. Moreover, the percentage of patients who adhered to the mouthwash regimen was 87% (range, 14% – 100%). There were no significant differences in the occurrence of other adverse events between the groups, and only one case of oral candidiasis was observed in the intervention group. Conclusions: Dexamethasone-based mouthwash safely reduced the incidence and severity of stomatitis in patients receiving chemotherapy (EC/TC) for early breast cancer. Clinical trial information: UMIN000030489.