Nanosized ceria (CeO₂) and a graphitic carbon nitride-loaded ceria (CeO₂/GCN) nanocomposite were synthesized using a straightforward and efficient method and characterized by XRD, FTIR, SEM, TEM, TGA, and BET analyses. These techniques confirmed that CeO₂ was effectively supported on the surface of GCN, with particle sizes of the CeO₂/GCN composite in the range of 10–15 nm and a pore size of 3.33 nm. The photocatalytic activity of the CeO₂/GCN nanocomposite and CeO₂ NPs in the degradation of methyl red dye under sunlight radiation was studied using UV–visible spectroscopy. A noticeable red shift in the CeO₂/GCN nanocomposite compared to pure CeO₂ NPs suggests a reduction in its band gap energy, calculated at 3.90 eV for CeO₂ NPs and 2.97 eV for the CeO₂/GCN nanocomposite. This band gap reduction enhances the photocatalytic degradation process, achieving a removal efficiency of 99.92% within a short irradiation time of 40 min for the CeO₂/GCN nanocomposite, compared to 69.47% for CeO₂ NPs. These findings indicate that graphitic carbon nitride significantly enhances the photocatalytic properties of CeO₂ NPs.