Recently, nitrate as one of the water pollutants has caused widespread concerns. The nitrite and ammonium of nitrate reduction products have also been regarded as contaminants. Thus, increasing the nitrate removal rate and improving the nitrogen selectivity simultaneously have attracted the attention of researchers. To remove the nitrate with high nitrogen selectivity from an aqueous solution, 2D g-C₃N₄ was intercalated into 3D TiO₂/Ti₃C₂ to fabricate the TiO₂/Ti₃C₂/g-C₃N₄ photocatalyst with 3D/2D ternary heterojunctions. 0.5 TiO₂/Ti₃C₂/g-C₃N₄ photocatalyst exhibited the highest nitrate conversion and nitrogen selectivity, which reached 93.03% nitrate conversion and 96.62% nitrogen selectivity after 40 min of irradiation. Moreover, its photocatalytic performance remained stable after 5 cycles. It was observed that the high concentration of nitrite was advantageous for improving the nitrogen selectivity. In TiO₂/Ti₃C₂/g-C₃N₄ composites, 3D Ti₃C₂ served not only as a template for preparing 2D g-C₃N₄, but also as a channel for the electron transfer between TiO₂ and g-C₃N₄. Photogenerated electrons in the conduction band of TiO₂ migrated to the valence band of g-C₃N₄ through Ti₃C₂. The corresponding holes remained on the valence band of TiO₂ and were captured by formic acid in an aqueous solution to generate reducing COO˙⁻. Then, COO˙⁻ in the solution and photoinduced electrons in the conduction band of g-C₃N₄ reduced nitrate to nontoxic and harmless nitrogen with high selectivity. Finally, the Z-scheme heterojunction was proposed over 0.5 TiO₂/Ti₃C₂/g-C₃N₄ photocatalyst on working for the photocatalytic nitrate reduction in water.