Description Datasets Datasets are obtained from X-ray diffraction (XRD), Rietveld analysis, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), nuclear magnetic resonance (MAS NMR) spectroscopy, absorption and photoluminescence spectroscopy, electrochemical impedance spectroscopy (EIS), chronoamperometry (CA) and chronopotentiometry (CP). Abstract Magnesium batteries offer promising potential as next-generation sustainable energy-storage solutions due to the high theoretical capacity of the magnesium metal anode. Facilitating dendrite-free operation of metal anodes necessitates the development of solid electrolytes (SEs) with high magnesium-ion conductivity. While the chalcogenide spinel MgSc2Se4 is predicted to exhibit high magnesium ion mobility, unequivocal experimental evidence for magnesium ion conduction beyond short-range motion is still missing. This study confirms magnesium-ion transport in MgSc2Se4 through two independent electrochemical methods: electrochemical deposition of magnesium metal and reversible magnesium plating/stripping cycling. To overcome the difficulty of measuring the ionic conductivity of the mixed conducting MgSc2Se4 spinel, a pure ion conducting interlayer is employed in a symmetric transference cell. This approach effectively suppresses the electron transport, allowing accurate characterization of the ionic conductivity. The experimental results confirm a low migration barrier (Ea) of (386 ± 24) meV for magnesium ion transport in MgSc2Se4 and demonstrate one of the best performances at room temperature among the reported inorganic magnesium solid electrolytes. The findings open a new door for exploring additional mixed magnesium ion conductors and highlight the potential of magnesium chalcogenide spinels as a promising class of magnesium solid electrolytes.