The Ni-doped Bi25FeO40 nanoparticles [Bi25 NixFe(1-x)O40, x = 0.1, 0.3, and 0.5] were synthesized by the hydrothermal method at a temperature of 180 °C. The effects of the Ni substitute on the structural, optical, magnetic, and photocatalytic performance have been investigated. The successful synthesis of the materials was confirmed by Rietveld's refined powder X-ray diffraction and Fourier transform infrared spectroscopy. Scanning electron microscopy together with energy dispersive X-ray spectroscopy (EDS) revealed the morphology of the synthesized nanomaterials as well as its chemical composition. The studied materials exhibited anomalous magnetic properties as evident from the vibrating sample magnetometry (VSM) analysis. These features display a weak ferromagnetic nature and demonstrate increased magnetization when doping elements are incorporated. Furthermore, Optical analysis revealed that bandgaps of Bi25NixFe(1-x)O40 were found to be increased from 2.06 eV to 2.31 eV with increasing Ni content. Additionally, the expansion of sillenite bismuth nickel ferrite's optical bandgap was further confirmed by the photocatalytic degradation of methylene blue as a model dye under the low-power white LED illumination. Lower bandgap undoped nanoparticles showed the highest photocatalytic efficiency, with a degradation rate of ∼43.11 % compared to higher bandgap doped nanoparticles, without adjustment of any reaction conditions such as pH or catalyst amount. The kinetic reaction rate of undoped Bi25FeO40 was roughly twice as fast as that of the material doped with 50 % Ni.