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Metal sulfides with diamond-like (DL) structures are systematically investigated in this study for nonlinear optical (NLO) applications in the mid-infrared (mid-IR) spectral region. All the selected materials are characterized by a 3D framework connected by various sulfide tetrahedral units, such as [PS 4 ], [SiS 4 ], [GeS 4 ], [GaS 4 ], [InS 4 ], [CdS 4 ], [ZnS 4 ], [LiS 4 ], and [AgS 4 ]. The linear and nonlinear optical properties of these materials are analyzed on the basis of first-principles calculations. It is concluded that it is relatively easy to achieve good balance between the bandgap and the NLO effect. Additionally, moderate birefringence Δn (∼0.03–0.10) is required for practical mid-IR NLO applications. In particular, several metal sulfides with normal DL and defect DL structures, exhibiting superior mid-IR NLO properties, are highlighted. These provide an instructive guide for the design and discovery of new materials with good mid-IR NLO performance. [ABSTRACT FROM AUTHOR]