Removal of nitrate from water has been receiving growing attention in water treatment. In this study, we report the photocatalytic denitrification (PCDN) by nonlinear optical (NLO) material, i.e. lithium niobate (LiNbO3). The hydrothermally synthesized LiNbO3 powder could achieve efficient denitrification in water, evidenced by 98.4% nitrate removal and 95.8% nitrogen selectivity at reaction time of 120 min and pH-neutral condition. Based on the first-order kinetics of PCDN, the kinetic constant for LiNbO3 is almost three times as that of conventional TiO2 (P25) under the same conditions. As suggested by the hole scavenger experiments, the LiNbO3 should proceed with photocatalytic reduction of nitrate through direct heterogeneous interaction with electrons at the conduction band of LiNbO3. This may represent a different mechanism from P25, where nitrate is mainly reduced by CO2•- radicals generated by the holes at the valence band. The unique second harmonic generation (SHG) effects of NLO materials enable them to produce more electrons and minimize the electron-hole recombination, which improves the efficiency and stability of the PCDN process. The current study provides a proof-of-concept demonstration of NLO photocatalytic material for more effective nitrate removal in water treatment.