Diesel engine technology is still the most effective solution to meet tighter CO2 regulations in the mobility and transport sector. In implementation of fuel‐efficient diesel engines, the poor thermal durability of lean nitrogen oxides (NOx) aftertreatment systems remains as one major technical hurdle. Divalent copper ions when fully exchanged into high‐silica LTA zeolites are demonstrated to exhibit excellent activity maintenance for NOx reduction with NH3 under vehicle simulated conditions even after hydrothermal aging at 900 °C, a critical temperature that the current commercial Cu‐SSZ‐13 catalyst cannot overcome owing to thermal deactivation. Detailed structural characterizations confirm the presence of Cu2+ ions only at the center of single 6‐rings that act not only as a catalytically active center, but also as a dealumination suppressor. The overall results render the copper‐exchanged LTA zeolite attractive as a viable substitute for Cu‐SSZ‐13.