Aim: This was a double-blind, placebo-controlled randomized study investigating whether melatonin can protect against radiation dermatitis in women receiving radiation therapy for primary breast cancer. Methods: Patients were included before radiation therapy and followed once weekly throughout treatment with a 3-week follow-up. Patients applied 1 g of cream to the irradiated skin twice daily, consisting of either 25 mg/g melatonin and 150 mg/g dimethyl sulfoxide, or placebo. Our outcomes were the Radiation Therapy Oncology Group's (RTOG) acute radiation morbidity scoring criteria for skin, a pixel analysis of erythema in clinical photographs, and patients’ use of corticosteroid cream. Outcomes were evaluated once weekly throughout the trial. The primary outcomes were RTOG-score and pixel analysis at 2 weeks follow-up. Secondary outcomes were the use of corticosteroid cream and analyses of RTOG-scores and pixel analyses throughout the trial. Results: Sixty-five patients were included, 17 dropped out, totaling 26 and 22 patients randomized to melatonin and placebo, respectively. RTOG-scores and pixel analyses at 2 weeks follow-up showed no difference p =.441 and p =.890, respectively). There was no difference in the use of corticosteroid cream (p =.055). Using logistic regression, the melatonin group had a higher likelihood of having a low RTOG-score (p =.0016). The logistic regression showed no difference between the groups for the pixel analyses. Conclusion: Our primary outcome showed no difference in RTOG-scores at 2 weeks follow-up, however, the RTOG-score over the entire duration of the study demonstrated a protective effect of melatonin. Further studies are warranted investigating higher doses of melatonin, and whether corticosteroids may influence the effect of melatonin cream against radiation dermatitis.