A series of Al_1−xCu_xTiO_3+δ (x = 0.2, 0.4, 0.6 and 0.8) (ACT) nanoparticles were synthesized via hydrothermal method using low operating temperatures. The diffraction patterns confirmed the presence of tetragonal structure including some secondary phases. The well-defined nanosized grains, and particles were detected in the surface morphology. The particle size distribution (PSD) showed the existence of smallest, and largest particles for x = 0.2–0.8. The presence of Al, Cu, Ti, and O was confirmed from energy dispersive spectral (EDS) analysis. The wide optical bandgap was noticed for all copper concentrations. Moreover, the photocatalytic activity was observed for all samples. The frequency dependence of dielectric constant, and dielectric loss was clearly discussed. The high dielectric constant, and loss were noticed for x = 0.8 sample, where the copper concentration is high. This can suggest the dielectric absorber applications. The space charge polarization mechanism, and relaxations were clearly understood using dielectric modulus, and impedance spectra. Cole–Cole plots conveyed the relaxation dynamics of x = 0.2–0.8.