Comprehensive Summary Oxychalcogenides have been drawn much attention to be explored as nonlinear optical (NLO) materials in recent years. To date, all the NLO oxychalcogenides crystallize in low dimensional structures. Herein, a new oxysulfide K2Ba0.5Ga9O2S13 was obtained via three‐dimensional (3D) {[Ga9O2S13]3–}∞ polyanionic framework. As the first oxychalcogenide with 3D NLO‐acitve polyanionic framework, K2Ba0.5Ga9O2S13 has a wide band gap of 3.72 eV, and exhibits a high laser‐induced damage threshold (LIDT, 17 × AgGaS2). It presents moderate phase‐matchable NLO response of 0.5 × AgGaS2 at 2.1 μm and 2 × KH2PO4 at 1.064 μm. The theoretical calculation results indicate that the NLO response is mainly contributed from [Ga(2)S4] unit‐built [Ga(2)3S10] windmill‐like motif. This work may stimulate the exploration of more NLO oxychalcogenides with balanced performances.