A new selenide, SnGa2GeSe6, in the AMIII2MIVQ6 family was synthesized for the first time by a high-temperature solid-state reaction. It crystallized in the non-centrosymmetric space group Fdd2 with cell dimensions of a = 47.195(9) Å, b = 7.5213(15) Å, c = 12.183(2) Å, and Z = 16. SnGa2GeSe6's crystal structure is characterized by a crisscross network of two types of infinite chains (i.e. the 1∞[GaSe3] chain and the 1∞[M3Se7] chain, where M represents the two metal sites randomly occupied by Ga and Ge atoms in a 1 : 1 ratio), which is similar to SnGa2GeS6 and diverges strongly from its Ba analogue owing to the substitution of Ba with Sn atoms that contain stereochemically active lone pair electrons. Careful experimental research has revealed that SnGa2GeSe6 exhibits an optical band gap of 1.98 eV and incongruent melting behavior. Furthermore, the second harmonic generation (SHG) intensity of the SnGa2GeSe6 powder sample is about 1.7 × AgGaS2 at a particle size of 150-200 μm with a 2 μm laser as the fundamental light.