This review highlights the significance of the Japan Diabetes Complications Study (JDCS), one of the earliest large-scale studies of people with type 2 diabetes outside Europe and the United States, in understanding type 2 diabetes mellitus among East Asian populations, particularly in Japan. Historically, large-scale clinical studies on type 2 diabetes mellitus have predominantly focused on Western populations, despite East Asians comprising the largest proportion of diabetic patients globally. The JDCS, which was initiated in 1996, enrolled 2,033 Japanese type 2 diabetes mellitus patients. It aimed to evaluate the effects of intensive lifestyle interventions on diabetic complications. The study demonstrated that lifestyle-focused interventions significantly reduced the risk of stroke and other complications compared to conventional treatment. Key findings of its sub-analyses include the unique characteristics of Japanese patients with type 2 diabetes mellitus, such as their lower body mass index (BMI) compared to Western counterparts and a stronger association between even modest BMI increases and beta cell dysfunction. Additionally, the JDCS provided insights into the risk factors for nephropathy, retinopathy, and macrovascular complications, emphasizing the importance of controlling blood pressure, glycemia, and lifestyle factors. The study also explored the impact of diet, exercise, and mental health on diabetic outcomes, revealing the protective effects of physical activity and a balanced diet, while highlighting the risks associated with high salt intake and depression. A risk prediction model tailored to Japanese patients was also developed. Overall, this study made a significant contribution to the evidence-based management of type 2 diabetes mellitus in East Asia.