Capacitance is generally determined by the porous microstructure, electron conduction and the synergy effect of active sites in the porous electrode. In this work, we grew centimeter-scale metallic porous GaN single crystals with conductivity up to 18 S/cm at room temperature. The Cu-catecholates (Cu–CAT) nanowire arrays were grown on porous GaN single crystal to form porous single-crystalline electrode with enhanced supercapacitor performance. The Cu–CAT/GaN single crystalline electrode exhibits specific capacitance of 216 F/g and normalized capacitance of 40 µF/cm². After 5000 cycles, it retains 80% of its initial capacitance. The porous single-crystalline GaN electrode has high porosity and excellent conductivity showing high surface capacitance.