The objective of the study was to evaluate the efficacy of oral magnesium supplementation in the improvement of metabolic syndrome (MetS) and its components. This is a randomized double-blind, placebo-controlled clinical trial that enrolled 198 individuals with MetS and hypomagnesemia who were randomly allocated to receive either 30 mL of magnesium chloride 5% solution, equivalent to 382 mg of elemental magnesium (n = 100), or placebo solution (n = 98), daily for 16 weeks. Serum magnesium levels <1.8 mg/dL defined hypomagnesemia. At final conditions, a total of 48 (48%) and 76 (77.5%) individuals had MetS in the magnesium and placebo groups (P = 0.01), respectively. At baseline, percent of individuals with 3, 4, and 5 criteria of MetS in the magnesium group were 60.0%, 37.0%, and 3.0%, respectively, and in the control group 55.1%, 35.7%, and 9.2%, respectively. Between basal and final conditions, changes in the components of MetS were significantly higher in the magnesium than placebo groups: -3.6 ± 3.3 mmHg, P = 0.001 for systolic blood pressure; -5.5 ± 1.7 mmHg, P = 0.005 for diastolic blood pressure; -12.4 ± 3.6 mg/dL, P < 0.005 for fasting glucose; -61.2 ± 24 mg/dL, P = 0.003 for triglycerides; and 0.9 ± 0.4 mg/dL, P = 0.06 for high-density lipoprotein cholesterol. Magnesium supplementation improves MetS by reducing blood pressure, hyperglycemia, and hypertriglyceridemia.