Use of the guanidinium iodide (GAI) cation is widely recognized as an interface engineering technique for perovskite solar cells that deliver stability improvements via defect passivation on surfaces and grain boundaries. However, a comprehensive understanding of the relationship between the structural and photophysical properties is lacking. Herein, GAI‐induced perovskite structural modifications, including derivative phases and underlying transitions, are detected in GAI surface‐treated Cs_0.07MA_0.14FA_0.79Pb(I_0.83Br_0.17)₃ through an analysis of X‐ray and electron diffraction and microscopy data. An optimum GAI solution concentration at 10 mg mL⁻¹ can eliminate excess PbI₂, improve crystallinity, and increase grain size of the as‐prepared perovskite films. However, a further increase to 20–40 mg mL⁻¹ induces new (FAPbI₃) _x (GA₂PbI₄) _x phases and a reduction in crystallinity and grain size. In addition, from confocal photoluminescence imaging, it is observed that 10 mg mL⁻¹ GAI also helps to remove the microscale spatial heterogeneities, demonstrating optimum device performance. These results show that understanding the impact on structure and microstructure of the selection and concentration of surface treatment agents is critical for the homogenization of perovskite optoelectronic properties and achieving efficient device.