In this study, we investigate the optical and structural properties of Cu-doped β-Ga₂O₃ nanostructures synthesized via a hydrothermal method, followed by annealing in ambient O₂. Different Cu doping concentrations (0, 1.6, and 4.8 at.%) are introduced to examine their effects on the crystal structure, chemical state, and optical bandgap of β-Ga₂O₃. X-ray diffraction (XRD) analysis reveals that the host β-Ga₂O₃ crystal structure is preserved at lower doping levels, whereas secondary phases (Ga₂CuO₄) appear at higher doping concentrations (4.8 at.%). X-ray photoelectron spectroscopy (XPS) confirms the presence of Cu²⁺ ions in both lattice substitution sites and surface-adsorbed hydroxylated species (Cu(OH)₂). The optical bandgap of β-Ga₂O₃ is found to decrease with increasing Cu concentration, likely due to the formation of localized states or secondary phases. These findings demonstrate the tunability of the optical properties of β-Ga₂O₃ via Cu doping, providing insights into the incorporation mechanisms and their impact on structural and electronic properties.