Two new halogenated selenites, namely, Pb₂Cd(SeO₃)₂X₂ (X = Cl and Br), were synthesized via mild hydrothermal reactions. The isomorphic compounds crystallized in a noncentrosymmetric (NCS) and chiral space group. Their three-dimensional (3D) structures were formed by alternating undulated [Pb₂(SeO₃)X]_∞⁺ cationic layers and flattened [Cd(SeO₃)X]_∞⁻ anionic layers. Thermal analyses revealed that the compounds can remain stable up to 500 °C. The SHG efficiencies of Pb₂Cd(SeO₃)₂X₂ (X = Cl and Br) are much higher than those of commercial KDP (KH₂PO₄), and they are phase-matchable. UV-vis-NIR diffuse reflectance spectra revealed that the two compounds exhibited wide optical band gaps. The laser damage thresholds of Pb₂Cd(SeO₃)₂X₂ (X = Cl and Br) were measured to be about 38.5 and 35.7 times higher than that of AGS (AgGaS₂). Their calculated birefringences were large enough to accomplish phase matching in SHG progression. The SHG density analyses disclosed that the lone pair electrons of the SeO₃ and PbO_nX_m units made major contributions to their nonlinear efficiencies.