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This study reports on highly efficient nonlinear optical ionic quinolinium crystals with various aromatic anions possessing different spatial volume for controlling the space‐filling characteristics and introducing halogen substituents having simultaneous electron‐withdrawing and electron‐donating effects. The quinolinium crystals with chloro and bromo substituents having simultaneous electron‐withdrawing and electron‐donating effects on anions exhibit perfect molecular ordering for optimizing the diagonal second‐order nonlinear optical response, while the fluoro substituent having relatively small electron‐donating effects on anions results in the absence of nonlinear optical response. Compared to widely used 4‐methylbenzenesulfonate, the chloro substituent on 4‐chlorobenzenesulfonate (CBS) results in decreasing π–π interactions between cations and anions. This leads on the one hand to an enhanced …