Using an ion-substitution strategy, herein, a new polar material, Pb₃(SeO₃)Br₄, with a greatly enhanced SHG response has been successfully designed and synthesized through a hydrothermal reaction. Pb₃(SeO₃)Br₄ crystallizes in the NCS space group P2₁2₁2₁ and consists of a three-dimensional framework formed by interconnecting one-dimensional chains, with a good thermal stability up to 230 °C. This compound exhibits a phase-matchable SHG response as strong as that of KH₂PO₄ (KDP) and a relatively wide mid-infrared (mid-IR) transparent window. Moreover, the optical band gap of Pb₃(SeO₃)Br₄ reaches about 3.35 eV, thus leading to a high laser damage threshold (LDT) of 67 MW cm⁻², which is over 12 times that of AgGaS₂ (<5 MW cm⁻²) measured under the same condition. All these findings suggest that Pb₃(SeO₃)Br₄ would be a candidate for an NLO material in the mid-IR region.