Two noncentrosymmetric borates, Na8MB21O36 (M = Rb and Cs), have been synthesized, structurally analyzed, and characterized for the first time. Their fundamental building block (FBB) is determined as the unprecedented [B21O36](9) units, which is cotemplated by four-connected [BO4](5) and five-connected [B5O11](7) clusters. To the best of our knowledge, the [B21O36](9) FBB has never been reported in borates. Besides, the structure comparisons among all the available anhydrous borates with high-polymeric FBBs were carried out to enlighten the investigation of B-rich borates.