The first tellurium­(II) thiourea sulfate semiorganic nonlinear optical (NLO) material, Te­(CS­(NH2)2)4SO4·2H2O, was successfully obtained using the slow evaporation technique. The structure of this metal-free crystal featured three types of NLO-active units, namely CS­(NH2)2, (SO4)2–, and (TeS4)6–, which synergistically contributed to its moderate second harmonic generation response (2.4 × KDP@1064 nm) and large birefringence (0.210@546.1 nm). Remarkably, this is the first example in which the quadrangular (TeS4)6– groups significantly contributed to the SHG effect and birefringence, and the first principles calculations revealed its special mechanism of NLO activity. These findings highlighted the exploration of semiorganic NLO materials and provide new opportunities for the discovery of new NLO crystals.