Four sodium‐based ternary IR nonlinear optical (NLO) materials, Na6MQ4 (M=Zn, Cd; Q=S, Se), were prepared through a high‐temperature flux method. The crystal structure of the compounds is built up of isolated [MQ4] tetrahedra and a 3D framework formed by the NaQn (n=4, 5) units. The two selenides, Na6MSe4 (M=Zn, Cd), as promising IR NLO materials, show moderate second‐harmonic generation (SHG) responses (0.9 and 0.5×AgGaS2) with good phase‐matching behavior, as well as high laser damage thresholds (2 and 1.9×AgGaS2). The two sulfides, Na6MS4 (M=Zn, Cd), exhibit higher laser damage thresholds (13 and 4×AgGaS2), but smaller SHG responses (0.3 and 0.2×AgGaS2). Theoretical calculations and statistical analyses indicate that the SHG effect and band gap in the compounds originate mainly from the distorted NaQ4 NLO‐active units with a short Na−S bond length, which provides a new insight into the design of novel IR NLO materials.