The objective of this study was to examine the changes of blood pressure and hemodynamic parameters after oral magnesium supplementation in patients with essential hypertension. The single-arm non-blinded intervention study comprised 48 patients (19 men; 29 women) whose antihypertensive therapy was not changed for at least one month. The participants were asked to consume (daily at home) 300 mg of oral magnesium-oxide supplementation product for one month and to have their blood pressure and hemodynamic parameters (thoracic fluid content, stroke volume, stroke index, cardiac output, cardiac index, acceleration index, left cardiac work index and systemic vascular resistance index, heart rate) measured in the hospital before and after the intervention. Measurements were performed with impedance cardiography. After magnesium supplementation, systolic and diastolic pressures were significantly decreased (mean ± standard deviation (SD)/mmHg/from 139.7 ± 15.0 to 130.8 ± 13.4 and from 88.0 ± 10.4 to 82.2 ± 9.0, respectively; both p < 0.001). The two significant hemodynamic changes were the decrease of systemic vascular resistance index (dyn s m2/cm5) and left cardiac work index (kg m/m²)/mean ± SD from 2319.3 ± 753.3 to 2083.0 ± 526.9 and from 4.8 ± 1.4 to 4.4 ± 0.9, respectively; both p < 0.05). The observed hemodynamic changes may explain lowering blood pressure after magnesium supplementation.