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Although the luminescent performance of organic–inorganic metal halides (OIMHs) have obtained significant advances, achieving intrinsic water‐stable OIMHs remain a substantial challenge due to the fragile ionic nature of hybrid the halide structure. To overcome these challenges, a structural design strategy is proposed that involves the use of highly hydrophobic cations as a protective layer to improve the water stability of OIMH. Herein, an aprotic trimethylsulfoxonium [TMSO]+ is selected as a hydrophobic cation and successfully assemble two new manganese based OIMHs of (TMSO)MnCl3 and (TMSO)MnBr3 through facile solid and liquid phase reaction methods. Remarkably, these halides exhibit strong red light emissions with high quantum yields recorded at 86.1% and 53.4%, respectively, originating from the octahedral [MnX6]4− based one‐dimensional (1D) [MnX3]− chain. Most significantly, these halides present extraordinary structural and luminescent stabilities toward continuous corrosion by humid air, water, and acid‐aqueous solution for more than one month, suggesting promising application prospects in extreme chemical environments. In‐depth Hirshfeld surface calculations demonstrate that the ultrahigh water‐stability benefits from the abundant hydrogen bonds and strong electrostatic interactions between [TMSO]+ and [MnX3]− ions, which provides an underlying insight into the stability mechanism. This water‐stability enhancement strategy represents a breakthrough structural engineering to rationally design more water‐stable OIMH. [ABSTRACT FROM AUTHOR]