A new class of 2D nanosheets of nitrogen-integrated phosphate-rich ammonium manganese phosphate hydrate, (NH₄MnPO₄·H₂O) (AMP), has been developed as pseudocapacitive electrode materials. The optimized electrodes exhibited device capacitances of 48.4 and 65.4 F/g for symmetric and asymmetric configurations, respectively. The devices showed excellent energy and power (e.g., 29.4 Wh/kg and 133 kW/kg for asymmetric cells) with extraordinary capacitance retention (e.g., >93%, 100 000 cycles at 5 A/g for asymmetric cells) that surpass those of most of the reported values. The huge pseudocapacitance of AMP is attributed to several factors, including the electroactive sites containing NH₄⁺ ions, the conductive inorganic layers, intercalated water interactions of Mn²⁺···H₂O, redox-active phosphate ions, and the 2D nanosheets. AMP-based all-solid-state flexible asymmetric devices exhibited >95% capacitance retention upon 1000 repetitive charge–discharge cycles. This study opens doors to elegant strategies of unlocking the rich physicoelectrochemical properties of 2D AMP for next-generation pseudocapacitors.