Magnesium (Mg) ions directly influence vascular tone and responsiveness and are cofactors for acetylcholine-induced endothelium-dependent relaxation. Alterations in extracellular Mg are able to modify the formation and release of nitric oxide (NO), altering arterial smooth muscle tone. Previous in vivo studies in humans have shown that parenteral or oral Mg supplementation increase endothelial-dependent vasodilation. The aim of the present study was to evaluate the effects of Mg oral supplementation on endothelial function in elderly diabetic and hypertensive subjects. Sixty elderly (≥ 65 years) diabetic patients were recruited (mean age: 71.1 ± 6.1 years; M/F: 35/25). Endothelial function, evaluated by non-invasive flow-mediated dilatation of the brachial artery, as well as anthropometric and laboratory data, including ionized Mg (Mg-ion), were measured in all patients before and after one-month. Thirty patients underwent oral Mg supplementation with 4.5 g/day of Mg pidolate (368 mg/day of Mg ion), while the rest were used as a control group. The usual management of diabetes and hypertension was not changed during the month of study participation for all the patients. In the group of patients that underwent Mg supplementation, Mg-ion concentration significantly increased from 0.42 ± 0.05 mmol/L to 0.49 ± 0.06 mmol/L; p < 0.05. Mg intervention resulted in a significant improvement of the post-ischemic endothelial-dependent flow-mediated dilation (from 3.3 ± 3.6% to 8.4 ± 3.9%; p < 0.05). No significant differences were found, either in ion-Mg or endothelial function, in the control group. In conclusion, the present study suggests that oral Mg improves endothelial function in diabetic elderly subjects.