Crystallizing in a noncentrosymmetric (NCS) structure is the essential requirement for a crystal to be second-order nonlinear optical (NLO) active. Here, a simple strategy of partial congener substitution is introduced to induce transformation of the known centrosymmetric K3Ga3Ge7Se20 (P2(1)/c) to the new isostructural NCS species K3Ga3(Ge6.17Sn0.83)-Se-20 (1) and K3Ga3 (Ge4.95Si2.05)Se-20 (2) (Pc). Their structures feature a {[Ga-3(Ge7-xMx)Se-20](3-)}(infinity) (M = Si, Sn) polyanionic framework built from the basic functional motif M'Se-4 (M' = Ga, Ge, M) tetrahedra, similar to but slightly distorted from that of K3Ga3Ge7Se20. Their NCS structures are verified by the NLO activities. The NLO response of 1 is similar to 1/3 times that of the benchmark AgGaS3 under a 2.1 mu m laser radiation, while 2 exhibits a weak effect, and both are NLO phase-matchable. Their optical band gaps are measured to be 2.02 and 2.12 eV, and they are transparent in the ranges of 0.61-25 and 0.58-25 mu m, respectively.