PURPOSE: The trial aimed to investigate the effectiveness of exogenous melatonin as an adjuvant to pregabalin for relief of pain in patients suffering from painful diabetic neuropathy (PDN). PATIENTS AND METHODS: This randomized, double-blind, placebo-controlled trial was carried out between October 2019 and December 2020 in an outpatient specialty clinic in Iran. One-hundred-three type 2 diabetic patients suffering from PDN were randomized into either the melatonin group (n = 52) or the placebo group (n = 51). Besides pregabalin at a dose of 150 mg per day, patients started with melatonin or an identical placebo, at a dose of 3 mg/day at bedtime for 1 week, which was augmented to 6 mg/day for further 7 weeks. The primary outcomes were changes in mean NRS (numerical rating scale) pain score from baseline to endpoint and responder rate (patients with a reduction of 50% and higher in average pain score compared with baseline). Secondary endpoints were changes in mean NRS pain-related sleep-interference score, overall improvement evaluated by Patient and Clinical Global Impressions of Change (PGIC, CGIC), and impact of the intervention on patient's Health-related quality of life (QOL). All analyses were conducted on an Intention-to-Treat (ITT) analysis data set. RESULTS: At the study endpoint, treatment with melatonin resulted in a considerably higher reduction in the mean NRS pain score in comparison with placebo (4.2 ± 1.83 vs. 2.9 ± 1.56; P-value < 0.001). In terms of treatment responders, a greater proportion of melatonin-treated patients satisfied the responder criterion than placebo-treated patients (63.5% vs. 43.1%). Melatonin also reduced pain-related sleep interference scores more than did placebo (3.38 ± 1.49 vs. 2.25 ± 1.26; P-value < 0.001). Further, at the endpoint, more improvement was also seen in terms of PGIC, CGIC, and Health-related QOL in patients treated with melatonin than placebo. Melatonin was also well tolerated. CONCLUSION: The present results showed that melatonin as an adjunct therapy to pregabalin might be helpful for use in patients with PDN. However, confirmation of these results requires further studies.