Two new rare-earth borates, MBaYB₆O₁₂ (M = Rb, Cs), which are the first reported compounds in M₂O-BaO-RE₂O₃-B₂O₃ (M = Rb, Cs; RE = rare-earth) quaternary systems, have been discovered through a high-temperature solution method and their structures were determined by single-crystal X-ray diffraction. These two compounds are isostructural with the centrosymmetric space group R (no. 148) and the fundamental building blocks are the B₃O₆ groups which are nearly parallel to each other. The spectral measurement shows that the cutoff edges of the two compounds are all down to the deep-ultraviolet (deep-UV) region (below 200 nm). And the experimental birefringence of RbBaYB₆O₁₂ and RbBaYB₆O₁₂ is about 0.12 at 589 nm. Moreover, first-principles theoretical studies were carried out to gain a better understanding of the structure–property relationships. The calculation results demonstrate that both the compounds have a large birefringence of 0.100 at 589 nm, which corresponds to the experimental value. Both experimental and theoretical studies suggest that MBaYB₆O₁₂ (M = Rb, Cs) are potential birefringent materials for UV and deep-UV applications.