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 (3) over bar (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.