Abstract: Bi₃O₃O₃SeBr was synthesized via traditional high temperature solid state reaction. Powder X-ray diffraction, DFT calculations, diffused reflectance spectroscopy and photoelectric measurements were used to characterize the crystal structure and physical properties of Bi₃O₃O₃SeBr. Experimental results show that the Bi₃O₃O₃SeBr crystallizes in space group P4/nmm with unit cell parameters a=3.922 84(6) Ǻ and c=20.238 14(3) Ǻ. Its crystal structure consists of Bi₂O₂ layers, Se^2- layers and Br- layers. The Se^2- layers and Br- layers are stacked between Bi₂O₂ layers, forming the structural units of Bi₂O₂Se and BiOBr. Bi₃O₃O₃SeBr can be seen to be composed of the structural units of Bi₂O₂Se and BiOBr sharing the Bi₂O₂ layers along the c direction. Bi₃O₃O₃SeBr is n-type semiconductor with narrow indirect band gap of 0.44 eV. The energy states at the top of the valence band are dominated by Se 4p states, while those at the bottom of the conduction band are dominated by Bi 6p states. Irradiated by visible light, Bi₃O₃O₃SeBr shows a reliable photo-electric response. Its photo current density is 0.70 μA/cm², superior to that of Bi₂O₂Se, indicating its prospect for application in photodetection.