A novel chiral orthoborate, KMgBO(3), has been successfully synthesized via a standard solid-state reaction, and the crystal structure has been determined from powder X-ray diffraction data. It crystallizes in the cubic chiral space group P2(1)3. Isolated [BO(3)](3-) anionic groups, which are the fundamental building units, are distributed around the 111 crystallographic direction in an axial C(3) symmetry, and the configuration of O(3) planes is helical, similar to the aplanar ClO(3) groups in NaClO(3), which indicates an optical activity in it and is validated by the circular dichroism spectrum. The chiral feature of the borate is unique, and because of the noncentrosymmetric structure, the compound exhibits nonlinear optical properties, which are validated by second harmonic generation powder measurement. It has a wide transparency, including the vacuum ultraviolet region. The relations between the structures of KMgBO(3) and NaClO(3) and structural conversion form KMgBO(3) to NaMgBO(3) are also discussed. The ionic radii, interatomic distance, and the rigid tilting of BO(3) groups are considered to be the main factors in the crystallization of different structures.