The deficiency of nonlinear optical (NLO) materials in the infrared region pushes continuous exploration of new ones with desired performances. In this work, a novel pentanary selenide KInSi1.32Sn0.68Se6 is discovered by using a high-temperature solid-state reaction. It crystallizes in the monoclinic noncentrosymmetric space group Cc featuring a 3D [(InSi1.32Sn0.68Se6)-]n polyanionic network, and its powder sample presents promising NLO behaviors, namely 1.3-times second-harmonic generation response and close laser-induced damage threshold than those of benchmark AgGaS2. Besides, it is phase-matchable and shows a wide transparent region. Theoretical calculation addressing its electronic structure and optical property is also performed.