Background: Magnesium, which is an essential trace element in bone metabolism, has been paid to great attention in recent years due to its osteogenic effect and critical role in bone mineralization process. Objective: Detection of the association between serum Mg concentration and clinical and radio-graphical findings in patients with knee OA and in apparently healthy controls. Subjects and method: A cross-sectional study conducted on 100 female patients with primary knee osteoarthritis (OA) diagnosed according to the American College of Rheumatology (ACR) clinical and radiological classification criteria for knee OA. In addition, 100 apparently healthy volunteers with no features of OA matched for age and sex worked as control group. All the study populations underwent bilateral weight bearing anteroposterior x-ray of the knee which was assessed according to the Kellgren and Lawrence (KL) grading system. The Western Ontario and McMaster Universities Arthritis Index (WOMAC) and Visual Analogue Scale (VAS) were used to assess pain severity and functional limitation. Blood samples had been collected for assessing serum Mg concentration by colorimetric method. Results: The mean serum Mg level in the OA group was 1.97 ± 0.17 mg/dl and the mean level of Mg in the control group was 2.09 ± 0.15 mg/dl with high statistically significant difference between the two groups (p < 0.001). Conclusion: Serum Mg concentration may have an inverse relationship with radiographic knee OA. Therefore, high level of Mg could possibly exert a protective role in the control of knee OA and can be used in its management.