Two polymorphs of a new barium borate, alpha- and beta-Ba-3[B10O17(OH)(2)], have been synthesized through hydrothermal reactions at 500 degrees C and 1000 bar and their structures determined by single-crystal X-ray diffraction. The 3D framework structure of the alpha-form is formed of two different fundamental building blocks (FBB) with the descriptors 4 Delta 6 square[(sic)(sic)]- <3 square>- <2 Delta square>1-<3 square>-<2 Delta square>1 and 2030:<020> <020>. The former FBB is unique and contains unusual edge-sharing B04 tetrahedra. The /3 form has a double layer structure which is formed of two stereoisomers of a pentaborate polyanion with the descriptor 2030:<6.20> <[120>. Within a double layer, one sheet composed of FBBs in 1 configuration is connected by sharing tetrahedral vertices to the other sheet of FBBs in d configuration. The a-form is the first compound with edge-sharing B04 tetrahedra synthesized in aqueous solution under hydrothermal conditions.