BaTiO₃ is a large band gap semiconductor, which only absorbs UV light in the solar spectrum. In this study, a series of BaTiO₃–graphene nanocomposites with different weight addition ratios of graphene oxide (GO) have been synthesized by a facile one-pot hydrothermal approach, during which the reduced graphene oxide and the intimate interfacial contact between BaTiO₃ nanoparticles and the graphene nanosheets were obtained. The as-prepared samples were characterized by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy (XPS), and Fourier transformed infrared (FTIR) spectroscopy. The photocatalytic activity of the obtained BaTiO₃–graphene composites for the degradation of methylene blue (MB) under visible light irradiation at λ ≥ 400 nm was investigated, and a higher photocatalytic activity than that of pure BaTiO₃ was observed. According to the experimental results, we suggest a new photocatalytic mechanism, in which the role of graphene in the BaTiO₃–graphene nanocomposites is to act as an organic dye-like photosensitizer for large band gap BaTiO₃. The photosensitization process of BaTiO₃ by graphene transforms the wide band gap BaTiO₃ semiconductor into visible light photoactivity in dye degradation. This reaction mechanism could widen the applications of BaTiO₃–graphene nanocomposites in solar energy conversion, promoting our thinking on the microscopic charge carrier transfer pathway connected to the interface between the graphene and the semiconductor.