Photocatalysis is attracting huge interest for addressing current energy and environmental issues by converting solar light into chemical energy. For this purpose, we investigated the effect of La³⁺ and Se⁴⁺ co-doping on the photocatalytic activity of BiFeO₃. BiFeO₃ and Bi_0.92La_0.08FeO₃ nanoparticles containing different Se⁴⁺ doping content (BiFe_(1−x)Se_xO₃, x = 0.0, 0.02, 0.05, and Bi_0.92La_0.08Fe_(1−x)Se_xO₃, x = 0.0, 0.02, 0.05, 0.075, 0.1) were synthesized by a double solvent sol–gel route. The co-doped nanoparticles were characterized by X-ray diffractometry (XRD), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), and UV-vis diffuse reflectance spectroscopy (DRS), and their photocatalytic activity was studied by the photocatalytic degradation of Congo Red (CR) in aqueous solution under different wavelengths of light illumination. The band-gap of the pure BiFeO₃ was significantly decreased from 2.06 eV to 1.94 eV. It was found that La³⁺ and Se⁴⁺ co-doping significantly affected the photocatalytic performance of pure BiFeO₃. Moreover, with the increment of Se⁴⁺ doping into Bi_0.92La_0.08FeO₃ up to an optimal value, the photocatalytic activity was maximized. In order to study the photosensitization process, photo-degradation of a colourless organic compound (acetophenone) was also observed. On the basis of these experimental results, the enhanced photocatalytic activities with La³⁺ and Se⁴⁺ co-doping could be attributed to the increased optical absorption, and efficient separation and migration of photo-generated charge carriers with the decreased recombination of electrons–holes resulting from co-doping effects. The possible photocatalytic mechanism of La³⁺ and Se⁴⁺ co-doped BiFeO₃ was critically discussed.