Several epidemiologic studies have found that magnesium ion (Mg²⁺) is related to obesity and type 2 diabetes mellitus. However, there have been almost no reports on the effects of a combination of excessive food intake and Mg²⁺ supplementation on metabolic syndrome and various blood tests values for diabetes mellitus. In this study, we investigated changes in body weight and blood test values for diabetes mellitus of Otsuka Long-Evans Tokushima Fatty (OLETF) rats, a model for human type 2 diabetes mellitus via metabolic syndrome, under conditions of combined excessive food intake and Mg²⁺ supplementation. The rats received Mg²⁺ supplementation by drinking magnesium water (Mg²⁺; 200 mg/l). No significant differences were observed in the levels of food or water intake between OLETF rats drinking purified water (PW) or magnesium water (MW). Type 2 diabetes mellitus with metabolic syndrome developed at 30 weeks of age, and the body weights and plasma insulin levels of OLETF rats at 60 weeks of age were lower than those of normal rats. The plasma glucose (PG) levels in 38-week-old OLETF rats drinking MW were significantly lower than in those of rats drinking PW, while the body weights and the levels of triglycerides (TG) and insulin of 38-week-old MW-drinking OLETF rats were significantly higher than those of their PW-drinking counterparts. On the other hand, the decreases in body weight and insulin levels in 60-week-old OLETF rats were suppressed by MW supplementation. The present study demonstrates that Mg²⁺ supplementation delays the development of diabetes mellitus in OLETF rats under conditions of excessive food intake. In addition, obesity and high blood TG levels were observed in OLETF rats receiving Mg²⁺ supplementation in conjunction with excessive food intake.