In the present work, gallium oxide nanoparticles  (nGa2O3) are synthesized via the thermal microwave combustion method, while nanocomposites of polyvinyl alcohol (PVA) polymer with various concentrations (0, 1, 2, 3, 4, and 5 wt%) of nGa2O3 are prepared by the casting technique. The structural characterization of nGa2O3, PVA, and films of PVA-Ga2O3 nanocomposites are studied using X-ray diffraction (XRD), High-resolution transmission electron microscopy (HRTEM), and Fourier-transform infrared (FTIR) spectroscopy. The HRTEM and XRD examinations showed that the prepared   nGa2O3 has an average crystallite size of ~ 5.6 nm and particle size of ~ 0.9 µm. The FTIR analysis reveals the occurrence of some interactions between  nGa2O3 and the functional groups of the PVA structure. On another side, the refractive index, absorption coefficient, and optical bandgap (Eg) were determined using the Wemple-DiDomenico single oscillator model. It was shown that Eg slightly reduced from 3.61 to 3.55 eV with increasing the  Ga2O3 content to 3 wt%, while raised again up to 3.58 eV for 5 wt%   Ga2O3. Other optical characteristics such as the optical density, extinction coefficient, optical susceptibility, thermal emissivity, optical sheet resistance for the PVA−Ga2O3 nanocomposites are investigated. The linear and nonlinear optical parameters together with their dependencies on the doping ratio reveals the qualification of PVA−Ga2O3 nanocomposites for nonlinear optical applications.