Mesoporous NiO single-crystalline nanosheet arrays grown on Ni foam with a thickness of about 5 μm have been fabricated by a facile hydrothermal approach and further investigated as a binder-free electrode for high-performance supercapacitors. Owing to the high conductivity of the single-crystalline NiO nanosheet arrays on the conducting substrate in combination with the large surface area provided by the mesoporous NiO nanosheets, the unique designed NiO nanosheet arrays on Ni foam have exhibited a high specific capacitance (674.2 F g−1 at a current density of 1 A g−1), good rate capability, and excellent cycling stability (93.5% capacitance retention after 5000 cycles). These results suggest that the single-crystalline NiO nanosheet array electrode is a promising candidate for high-performance supercapacitors and the rational design of this unique binder-free electrode demonstrated in this work provides a new and facile approach to fabricate other transition metal oxide arrays for electrochemical energy storage.