<p>Recently, green synthesis approaches for metal oxide nanoparticles have gained significant attention as viable alternatives to conventional methods owing to their simplicity, cost-effectiveness, and environmental sustainability. The present study aims to synthesize cerium oxide nanoparticles (CeO₂ NPs) via a hydrothermal-assisted green route using <i>Cannabis indica</i> leaves extract, with varying extract-to-precursor volume ratios. As-synthesized CeO₂ NPs are characterized using X-ray diffraction, Raman spectroscopy, field emission scanning electron microscopy, energy-dispersive X-ray analysis, dynamic light scattering, and diffuse reflectance spectroscopy. It is observed that both the crystallite size and optical band gap of the synthesized CeO₂ NPs depend on the volume of <i>Cannabis indica</i> extract. XRD and Raman spectroscopic analyses confirmed the formation of polycrystalline CeO₂ NPs featuring a cubic fluorite crystal structure. Morphological analysis indicated the formation of composed NPs of diverse sizes in spherical and semi-spherical geometry. The optical band gap is predicted from diffuse reflectance by the Kubelka–Munk function and Tauc’s plot. The photocatalytic performance of the chosen CeO₂ NPs is assessed by monitoring the solar-light-induced degradation of methylene blue and methyl orange dyes. The degradation process under optimal conditions predominantly obeys pseudo-first-order kinetics. Furthermore, a 1&#xa0;mg/mL dosage of CeO₂ NPs exhibits higher degradation efficiency for 5&#xa0;ppm dye solutions under visible light irradiation.</p> Graphical Abstract <p></p>