<p>We developed new organic π-conjugated salt crystals with non-centrosymmetric alignment of styryl fluoro-benzothiazolium cationic chromophores, resulting in top-level off-diagonal nonlinear optical susceptibility. The newly designed organic salt crystals consist of nonlinear optical cationic chromophore, 5-fluoro-2-(4-(2-(hydroxymethyl)pyrrolidin-1-yl)styryl)-3-methylbenzothiazol-3-ium (PB5FB) and molecular anion, 4-methoxybenzenesulfonate (MBS). PB5FB-MBS crystals with monoclinic <i>P</i>2₁ space group symmetry exhibit non-centrosymmetric molecular alignment of PB5FB cationic chromophores. In particular, the molecular ordering angle of PB5FB-MBS crystals is close to the optimal angle for maximizing the off-diagonal optical nonlinearity. This results in a very large effective first hyperpolarizability of up to ~ 100 ⋅ 10^− 30 esu, one of the highest off-diagonal susceptibility values for organic crystals. Interestingly, PB5FB-MBS crystals with the methoxy substituent on anion exhibit substantially different absorption spectrum in the terahertz (THz) frequency range of 12–32 THz compared to analogous crystals with the methyl substituent on anion. This is attributed to remarkably different molecular orientation of anions with respect to cations, intermolecular interaction characteristics, and Hirshfeld volume of anions in the crystalline state. Therefore, the newly developed PB5FB-MBS crystals exhibit high potential for diverse nonlinear optical applications.</p> Graphical abstract <p></p>