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A novel chiral orthoborate, KMgBO3, 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 P213. Isolated [BO3]3- anionic groups, which are the fundamental building units, are distributed around the (111) crystallographic direction in an axial C3 symmetry, and the configuration of O3 planes is helical, similar to the aplanar ClO3 groups in NaClO3, 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 KMgBO3 and NaClO3 and structural conversion form KMgBO3 to NaMgBO3 are also discussed. The ionic radii, interatomic distance, and the rigid tilting of BO3 groups are considered to be the main factors in the crystallization of different structures. [ABSTRACT FROM AUTHOR]