This manuscript aims to investigate the structural, thermal, and dielectric properties of polyester/Ni0.5Zn0.5Fe2O4 nanocomposites. The synthesized samples were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR), field emission scanning electron microscope, transmission electron microscope (TEM), thermogravimetric analysis (TGA), and dielectric measurements. The XRD patterns confirmed the existence of both polyester and Ni0.5Zn0.5Fe2O4 nanoparticles peaks. FTIR analysis confirmed the incorporation of Ni0.5Zn0.5Fe2O4 nanoparticles into the polyester matrix in agreement with XRD results. SEM micrographs showed distribution of nanoparticles inside the polymer and TEM image showed a little agglomeration of Ni0.5Zn0.5Fe2O4 with flake-like micrometer-sized particles. The incorporation of nanofiller into the polymer increases the thermal stability as the weight losses reduced with nanoparticles content. Dielectric analysis depicted the enhancement in both real and imaginary parts of permittivity when nanoparticles concentration increased. Additionally, the AC conductivity increased while the electric impedance decreased with increasing Ni0.5Zn0.5Fe2O4 concentration.