Experimental studies are carried out on novel sodium ion conducting, gel polymer electrolyte nanocomposites based on poly(methyl methacrylate) (PMMA) and dispersed with silica nanoparticles. The nanocomposites are obtained in the form of free-standing transparent films. A gel electrolyte with ∼4wt.% SiO2 offers the maximum electrical conductivity of ∼3.4×10−3 Scm−1 at ∼20°C with good mechanical, thermal and electrochemical stability. Physical characterization by X-ray diffraction, Fourier transformed infra-red and scanning electron microscopy is performed to examine ion/filler-polymer interaction and the possible changes in the texture of the host polymer due to liquid electrolyte entrapment and the dispersion of SiO2 nanoparticles. The temperature dependence of the electrical conductivity is consistent with an Arrhenius-type relationship in the temperature range from 25 to 75°C. Sodium ion conduction in the gel electrolyte film is confirmed from cyclic voltammetry and transport number measurements. The value of the sodium ion transport number ( t N a + ) of the undispersed gel electrolyte is ∼0.23 and it is almost unaffected due to the dispersion of SiO2 nanoparticles. The effect of SiO2 dispersion on ionic conduction is described in terms of anion–filler surface interaction.