Two new isostructural metal chalcogenides, K2BaSnS4 and K2BaSnSe4, have been isolated for the first time. They feature the intriguing infinity 1[BaSnQ(4)](2-) (Q = S, Se) tunnel structures which are separated by K+ cations. Experiments combined with theoretical calculations demonstrate that K2BaSnS4 possesses a large energy gap (3.09 eV), which is conducive for large laser-induced damage thresholds (LDTs), and maintains a relatively moderate SHG response (0.5 X AgGaS2) in the meanwhile. Moreover, K2BaSnS4 possesses good phase-matchability, which stems from its suitable birefringence ([email protected] mu m).