BACKGROUND: The incidence of both type 2 diabetes (T2DM) and cancer is increasing worldwide, making these diseases a global health problem along with increasing healthcare expenditures. The current therapeutic approaches for treating these multifactorial diseases are far from satisfactory. As increasing evidence shows beneficial effects of melatonin (MLT) on typical pathological changes occurring during the development of T2DM and cancer, the present review focuses on molecular aspects of antidiabetic and anticancer activities of MLT and, moreover, discusses several future directions of research regarding MLT application as potential therapeutic agent. METHODS: Critical literature analysis in PubMed central combined with personal expertise. RESULTS: Numerous in vitro and in vivo studies have revealed that MLT possesses a number of antidiabetic health benefits by diminishing hyperglycemia, insulin resistance, oxidative stress, and inflammation through modulating various intracellular signaling pathways or other targets involved in the pathophysiology of this disease. Mounting evidence also indicates that MLT exhibits multi-targeted anticancer effects in numerous human malignancies, mainly resulting from its ability to modulate several signal transduction pathways associated with cell survival, proliferation, and apoptosis. Furthermore, beneficial synergistic action of MLT with chemotherapy and radiotherapy has also been observed. Importantly, no adverse outcomes have been found from the clinical use of MLT, which highlights its therapeutic usefulness, either alone or in combination with other conventional therapies, in cancer treatment. CONCLUSION: The findings described in this review suggest that MLT may confer potential benefits to human health, particularly in respect to T2DM and cancer.