Optoelectronic materials with excellent birefringent properties are of significant importance in the fields of optical communications and laser technology. Recently, rare earth (RE) chalcogenides with anisotropic [RESn] groups have been proven to be high-performance infrared birefringent materials. Herein, we demonstrate that the addition of planar groups can further increase the birefringence in RE chalcogenides, as realized by incorporating planar [AgS3] groups into the RE-I-IV-S4 family for the first time. The newly obtained LaAgSiS4 compound shows higher polarity anisotropy than its homologue LaLiSiS4 and LaKSiS4, which resulted in a larger birefringence (0.12@600 nm) at least twice as large as that of the latter two compounds (0.05/0.06@600 nm). The structure-property relationship of LaAgSiS4 was investigated through structural analysis and first-principles calculations. The results indicate that the increased optical birefringence in LaAgSiS4 originates from the synergic effects of the distorted [LaSn] polyhedra and planar [AgS3] triangles. This work provides an effective strategy for enhancing optical birefringence in IR chalcogenides.