Standard high-temperature solid-state reactions of NaCl, Nb2O5, and SeO2 resulted in two new sodium selenites containing a second-order Jahn-Teller (SOJT) distorted Nb5+ cation, namely, Na2Nb4O7(SeO3)(4) (P (1) over bar; 1) and NaNbO(SeO3)(2) (Cmc2(1); 2). Compound 1 exhibits an unusual 3D [Nb4O7(SeO3)(4)](2-) anionic network composed of 2D [Nb4O11(SeO3)(2)](6-) layers which are further bridged by additional SeO32- anions via corner sharing; the 2D [Nb4O11(SeO3)(2)](6-) layer is formed by unusual quadruple [Nb4O17](14-) niobium oxide chains of corner-sharing NbO6 octahedra being further interconnected by selenite anions via Nb-O-Se bridges. The polar compound 2 features a 1D [NbO(SeO3)(2)](-) anionic chain in which two neighboring Nb5+ cations are bridged by one oxo and two selenite anions. The alignments of the polarizations from the NbO6 octahedra in 2 led to a strong SHG response of similar to 7.8 x KDP (similar to 360 x alpha-SiO2), which is the largest among all phases found in metal-Nb5+-Se4+/metal-Nb5+-Te4+-O systems. Furthermore, the material is also type I phase matchable. The above experimental results are consistent with those based on DFT theoretical calculations. Thermal stabilities and optical properties for both compounds are also reported.