There are limited and conflicting data from clinical trials concerning the beneficial effects of magnesium supplementation on diabetic patients. We investigated the effects of magnesium supplementation on metabolic control and insulin sensitivity in type 2 diabetic patients with normomagnesemia. A total of 98 normomagnesemic subjects with type 2 diabetes were enrolled in a randomized, crossover, double-blind, placebo-controlled trial. Participants were randomly assigned to receive magnesium lactate (360 mg elemental magnesium) or placebo for three months, followed by a three-month washout period. Treatment assignments were then reversed over an additional three months of follow-up. The primary endpoint was a reduction in fasting glucose and HbA1c. A total of 56 subjects completed the follow-up in the magnesium and placebo supplementation groups. Urinary magnesium excretion was increased following magnesium supplementation in the intervention group compared with the placebo group (p = 0.0002). Fasting glucose, HbA1c, insulin and HOMA-IR, as well as lipid profile, did not change significantly during treatment. We concluded that magnesium supplementation does not improve metabolic control or insulin sensitivity in diabetic subjects with normomagnesemia.