AbstractIt is important to establish and clarify the relationship between stereochemically active lone pairs and birefringence, since it is one of the significantly effective routes to explore birefringent crystals by introducing Sn‐centered polyhedra with stereochemically active lone pairs. Herein, four tin(II)‐based ternary halides A₃SnCl₅ and ASn₂Cl₅ (A=NH₄ and Rb) have been synthesized successfully. The experimental birefringence of Rb₃SnCl₅ and RbSn₂Cl₅ is larger than or equal to 0.046 and 0.123@546 nm, respectively. Through investigating the alkali or alkaline‐earth metal tin(II)‐based ternary halides, the structure‐performance relationship has been concluded between stereochemically active lone pairs and optical anisotropy. It is beneficial to the analysis and prediction of birefringence in tin‐based halides and provides a guide for exploring tin(II)‐based optoelectronic functional materials.