We investigated the effects of silica(SiO2) and clay nanoparticles on the tensile properties of poly(vinyl alcohol) (PVA) hydrogels containing borax. Clay/PVA/borax hydrogels possessed comparatively high tensile strength even at low concentrations of clay, but the mechanical performance drastically became worse at high concentrations. On the contrary, the tensile strength of SiO2/PVA/borax hydrogels increased with increasing SiO2 concentrations under high elongation. In these hydrogels, multiple crosslinking such as physical interactions between clay and PVA, complexations between PVA (or SiO2) and borate, microcrystals of PVA produced a tough polymer network. Besides, the synergistic effects of clay and SiO2 nanoparticles on the mechanical performance of the PVA/borax hydrogels were confirmed, i.e., both extensibility and modulus were largely improved compared to clay/PVA/borax or SiO2/PVA/borax hydrogels. Synchrotron small-angle X-ray scattering analysis revealed that the addition of both clay and SiO2 nanoparticles suppressed inhomogeneities in the composite gel. Display omitted •Clay/PVA/borax gels possessed high tensile strength at low clay concentrations.•SiO2/PVA/borax gels possessed high elongation even at high SiO2 concentrations.•The addition of SiO2 and clay to PVA/borax gels caused high elongation and strength.•Combined use of nanoparticles and borax was effective for production of tough polymer gels.