An alkaline-earth-metal iodate fluoride, Ba(IO3)F (1), was discovered by introducing F atoms into alkaline-earthmetal iodates via a hydrothermal method. The pseudo-two-dimensional (2D) [BaF](+) layers and [IO3](- )anionic groups built the crystal structure, which obviously exhibited large anisotropies. The birefringence of 1 was initially tested to be 0.1256 at 589.3 nm. Besides, compared with most reported metal iodate crystals, 1 possesses a large band gap (E-g = 4.32 eV). Theoretical calculations proved that the excellent optical characteristics can be attributed to [IO3](- )anionic groups and [BaO4F4](10- )polyhedra.