Due to the definitive known effect of magnesium on insulin resistance and the fact that insulin resistance is a main etiology in polycystic ovary syndrome (PCOS), it is assumed the use of magnesium supplements can improve insulin resistance, lipid profiles, and glucose and thus may also play a role in improving the clinical condition of patients with PCOS. We aimed to assess the effects of magnesium supplements on anthropometric, clinical, and metabolic parameters in women suffering from PCOS. This triple-blind randomized clinical trial study was conducted on women aged 15-35 years with PCOS. The patients were randomly assigned to receive a magnesium oxide supplement (250 mg/day for 2 months) or a placebo. The study parameters were evaluated and compared between two groups before as well as 2 months and 5 months after the initial assessment. In total, 40 cases (20 in each group) were recruited in the study. A significant reduction in the serum insulin level (P-value = 0.036) and insulin resistance (p-value = 0.032) was observed in the case group. Prescribing magnesium supplements could also lead to lowering total cholesterol, low-density lipoprotein, and fasting blood sugar along with increasing the level of high-density lipoprotein. We could not find any significant difference in anthropometric parameters as well as the mean systolic and diastolic blood pressures before and after intervention between the two groups. Although the rate of oligomenorrhea significantly decreased in the two study groups, it was no different across the two groups before and also after the intervention. The use of magnesium supplements in patients with PCO, regardless of the etiology or progression of the disease, can greatly improve the metabolic status of these patients by improving insulin resistance and modulating the level of lipid profile.