Circadian- and sleep disturbances may be central for understanding the pathophysiology and treatment of depression. The effect of melatonin on depression/depressive symptoms has been investigated previously. This systematic review assesses the current evidence of a therapeutic- and prophylactic effect of melatonin in adult patients against depression or depressive symptoms. A search was performed in The Cochrane Library, PubMed, EMBASE and PsycINFO for published trials on November 14th 2013. Inclusion criteria were English language, RCTs or crossover trials. Our outcome was measurement of depression/depressive symptoms with a validated clinician-administered or self-rating questionnaire. PRISMA recommendations were followed and the Cochrane risk-of-bias tool used. Ten studies in 486 patients were included in the final qualitative synthesis and four studies, 148 patients, were included in two meta-analyses. Melatonin doses varied from 0.5–6 mg daily and the length of follow-up varied from 2 weeks to 3.5 years. Three studies were done on patients without depression at inclusion, two studies in patients with depression and five studies included a mixture. Six studies showed an improvement in depression scores in both the melatonin and placebo groups but there was no significant difference. One study showed a significant prophylactic effect and another found a significant treatment effect on depression with melatonin compared to placebo. The two meta-analyses did not show any significant effect of melatonin. No serious adverse events were reported. Although some studies were positive, there was no clear evidence of a therapeutic- or prophylactic effect of melatonin against depression or depressive symptoms.