Infrared nonlinear optical (IR NLO) materials play a pivotal role in laser technology, yet achieving a balance between a wide band gap and a strong second-harmonic generation (SHG) response remains challenging. Here, we report the rational design and synthesis of ZnHg2P2S8, a novel diamond-like (DL) chalcogenide developed through a partial congener substitution strategy. The precise partial substitution of Zn2+ for Hg2+ in the normal [Hg1S4] tetrahedra, while preserving the structurally distorted [Hg2S4] units critical for NLO effects, simultaneously enhances the band gap to 2.91 eV (compared to 2.77 eV in the parent compound Hg3P2S8) and maintains a strong SHG response of 1.5 × AgGaS2. First-principles calculations elucidate the structure-property relationship, showing that the preserved [Hg2S4] motifs dominate the SHG effect, while the introduction of Zn enlarges the band gap. This work not only presents an exceptional IR NLO candidate overcoming the band gap-NLO efficiency paradox, but also establishes a general design strategy for developing high-performance IR NLO materials through targeted structural engineering.