A novel noncentrosymmetric (NCS) polar fluoride sulfate, CsSbF<sub>2</sub> SO<sub>4</sub> , was obtained by ionothermal synthesis. A meticulously designed co-substitution approach was used to successfully replace the [TiO<sub>6</sub> ]<sup>8-</sup> and [PO<sub>4</sub> ]<sup>3-</sup> functional groups in KTiOPO<sub>4</sub> (KTP) with [SbO<sub>4</sub> F<sub>2</sub> ]<sup>7-</sup> and [SO<sub>4</sub> ]<sup>2-</sup> units, respectively. The structure of CsSbF<sub>2</sub> SO<sub>4</sub> features a pseudo-3D framework consisting of interconnected 1D [SbF<sub>2</sub> O<sub>2</sub> SO<sub>4</sub> ]<sup>5-</sup> chains of corner-sharing [SbO<sub>4</sub> F<sub>2</sub> ]<sup>7-</sup> octahedra and [SO<sub>4</sub> ]<sup>2-</sup> tetrahedra. The title compound exhibits a sharply enlarged band gap compared to its parent compound, KTP, benefitting from the introduction of F<sup>-</sup> ions and the displacement of Sb<sup>3+</sup> cations. Second harmonic generation (SHG) measurements manifested that CsSbF<sub>2</sub> SO<sub>4</sub> is phase-matchable and revealed a strong SHG response of about 3.0 KH<sub>2</sub> PO<sub>4</sub> (KDP), which is the highest value reported for any metal sulfate reported to date. The reported fluoride sulfate is a promising near ultraviolet (UV) nonlinear optical (NLO) material.