Nickel oxide (NiO) is a promising electrochemical material owing to its high theoretical specific capacitance, environmentally benign nature, and low cost, and can be synthesized easily by various strategies. However, the poor cycling stability of NiO hinders its potential for next generation high performance energy storage applications. In this work, we demonstrate that two-dimensional (2D) NiO nanoflake arrays possess ultrathin thickness and abundant nanoscale pores vertically grown on the surface of three-dimensional nickel foam via a solvothermal reaction followed by sintering in air. Transmission electron microscopy shows that the 2D NiO nanoflakes are as thin as ∼7 nm and possess ample pores (<10 nm). The outstanding cycling stability is enabled by the unique porous structure, which not only reduces diffusion resistance of electrolytes in rapid redox reactions but also preserves mechanical integrity during prolonged charging/discharging. The 2D ultrathin porous NiO nanoflakes electrode exhibits remarkably high specific capacitance (2013.7 F g-1 at 1 A g-1 and 1465.6 F g-1 at 20 A g-1) and excellent cycling ability (100% capacitance retention over 5000 cycles). An asymmetric supercapacitor (ASC) operating at 1.5 V is assembled using ultrathin porous NiO nanoflakes and reduced graphene oxide (rGO) as positive and negative electrodes, respectively. The NiO//rGO ASC delivers a high specific capacitance of 145 F g-1 at 1 A g-1 with a high energy density of 45.3 W h kg-1 at a power density of 1081.9 W kg-1 and outstanding cyclic stability (91.1% capacitance retention after 5000 cycles). These promising results open up a pathway for developing advanced electrode materials for energy storage devices.