Diabetes mellitus (DM) is chronic disease characterized by hyperglycemia and insulin resistance caused by dysfunction of pancreatic β cells. Over the past few decades, epidemiological studies have suggested that dietary long-chain polyunsaturated fatty acids such as docosahexaenoic acid and eicosapentaenoic acid decrease the risk of metabolic diseases including DM. The mechanisms underlying the therapeutic efficacy of dietary long-chain polyunsaturated fatty acids in treating DM have been partly revealed. In this review, the authors describe the antidiabetic effects of long-chain polyunsaturated fatty acids and also discuss their possibilities as therapeutics for DM in the light of recent findings.