<p>Lead-free inorganic perovskites, such as Cs₃Fe₂Cl₉, have emerged as promising alternatives to address the challenges of toxicity and instability issues. However, the wide bandgap of Cs₃Fe₂Cl₉ limits its applicability in some applications. In this study, we systematically investigate the influence of ruthenium (Ru) doping on the structural and optical properties of Cs₃Fe₂Cl₉ perovskite with the various doping concentrations (0%, 5%, 20%, 50%, 70% and 100%). Comprehensive characterization was performed through X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), UV–Vis spectroscopy, and scanning electron microscopy (SEM). Structural analysis revealed a phase transition from Cs₃Fe₂Cl₉ (hexagonal structure) at 0% Ru doping to Cs₂RuCl₆ (cubic structure) at 100% Ru doping. Notably, ruthenium was successfully replaced into the pristine Cs₃Fe₂Cl₉ perovskite due to the similar ionic radii of Fe and Ru. Optical measurements confirmed a significant bandgap reduction from 2.29&#xa0;eV for pristine Cs₃Fe₂Cl₉ to 1.79&#xa0;eV at 100% Ru doping, correlating with the formation of single crystals. This tunable bandgap highlights the potential of Ru-doped Cs₃Fe₂Cl₉ for next-generation optoelectronic applications, such as photovoltaics and photodetectors.</p>