Background: Depression following acute coronary syndrome is prevalent and associated with increased mortality and morbidity. Melatonin may function as a primary prophylactic antidepressant substance and alleviate depressive symptoms. The study was undertaken to determine if melatonin administered following an acute coronary syndrome (ACS) could prevent development of depression. Methods: The study was a double-blinded, placebo-controlled, multicenter, randomized clinical trial performed in five primary care cardiology departments at Zealand, Denmark. Included patients were adults patients, free of depression at baseline, included at the latest 4 weeks after acute coronary syndrome. Twenty-five mg melatonin or placebo was administered 1 h before participants’ bedtime for 12 weeks. The primary outcome is Major Depression Inventory (MDI) measured every two weeks throughout the trial. Incidence of depression was apriori defined as MDI score ≥ 21 during the trial. Reported exploratory outcomes were patterns of dropout and safety outcomes. Results: 1220 patients were screened and 252 participants were randomized in a 1:1 ratio. Baseline MDI score in the melatonin and placebo group were, respectively, 6.18 (CI 5.32–7.05) and 5.98 (CI 5.19–6.77). No significant intergroup differences were found during the study in the intention-to-treat analysis or per-protocol analysis. Cumulative events of depressive episodes during the 12 weeks were six in the melatonin group and four in the placebo group. A significant drop in depressive symptoms were present throughout the study period. No intergroup differences were present in dropouts or adverse events. Conclusions: Melatonin showed no prophylactic antidepressant effect following acute coronary syndrome. The non-significant results might be due to a type II error or melatonin might not be able to prevent development of depressive symptoms following ACS.