Membrane‐based separation technologies have attracted significant interest from various industries owing to their high process efficiency. However, the wider applications of conventional polyamide (PA) thin‐film composite (TFC) membranes are limited by their poor pH stability and low cation selectivity, necessitating the development of membranes with advanced chemistries. Herein, an extreme pH‐resistant, highly cation‐selective TFC membrane is fabricated by synthesizing a crosslinked poly(quaternary ammonium) (PQA) selective layer on a polyethylene support via Menshutkin reaction‐based interfacial polymerization (Men‐IP). The Men‐IP process produces a thin, densely crosslinked, and positively charged PQA permselective layer without hydrolysis‐prone functional groups. The fabricated PQA membrane features a highly selective molecular density that significantly exceeds those of previously reported membranes with non‐PA chemistries. Moreover, the PQA membrane exhibits remarkably high rejection (>90%) and selectivity for divalent cations owing to the exceptionally strong positive charge imparted by its abundant cationic QA groups. More importantly, the PQA membrane displays ultrahigh pH stability under both extremely acidic (1.5 m H2SO4) and alkaline (5 m NaOH) conditions for 28 days. No other membrane reported in the literature demonstrates such excellent pH stability. This strategy opens a new route for fabricating highly selective membranes that can be used in harsh pH environments. A poly(quaternary ammonium) (PQA)‐based nanofiltration membrane with ultrahigh pH stability and excellent cation selectivity is fabricated via Menshutkin reaction‐based interfacial polymerization. A densely crosslinked, highly positively charged PQA layer leads to remarkably high cation selectivity for divalent cations. Furthermore, the designed PQA chemistry with no hydrolysis‐prone functional groups displays unprecedentedly high stability in extreme pH environments.