Experimental investigations are performed on novel magnesium ion-conducting gel polymer electrolyte nanocomposites based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP), dispersed with nanosized magnesium oxide (MgO) particles. The nanocomposite materials are in the form of free-standing films. Various physical and electrochemical analyses demonstrate promising characteristics of these films, suitable as electrolytes in rechargeable magnesium batteries. The optimized material with 3wt.% MgO offers a maximum electrical conductivity of ∼8×10−3 Scm−1 at room temperature (∼25°C) with good thermal and electrochemical stabilities. The ion/filler–polymer interactions and possible conformational changes in host polymer PVdF-HFP due to the liquid electrolyte entrapment and dispersion of nanosized MgO are examined by Fourier transform infrared (FTIR), X-ray diffraction (XRD) and scanning electron microscopic (SEM) methods. The Mg2+ ion conduction in the gel film is confirmed from the cyclic voltammetry, impedance spectroscopy and transport number measurements. The Mg2+ ion transport number (t +) is enhanced substantially and found to have a maximum of ∼0.44 for the addition of 10wt.% MgO nanoparticles. The enhancement in t + is explained on the basis of the formation of space-charge regions due to the presence of MgO:Mg2+-like species, that supports Mg2+ ion motion.