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β-Co(OH)2 nanosheets were prepared in NaNO3 solution by cathode glow discharge electrolysis (CGDE) technique, in which Co sheet and Pt needle were regarded as anode and cathode, respectively. The formation mechanism of β-Co(OH)2 was discussed in detail, and the electrochemical performance was investigated as an electrode material for the pseudocapacitor. The results showed that the obtained β-Co(OH)2 has a single phase of the hexagonal brucite-like nanosheets with an angle of adjacent edges of 120°, side length of 400–500 nm, and thickness of 10-20 nm. The average pore size and specific surface area are 33.1 nm and 122.1 m2 g−1, respectively. The β-Co(OH)2 nanosheets exhibit a high specific capacitance of 335 F g−1 and excellent capacity retention of 77.1% after 3000 charge-discharge cycles at 1 A g−1. CGDE offers a simple, fast, highly effective, and green strategy for the preparation of β-Co(OH)2 nanosheets. The prepared β-Co(OH)2 nanosheets can be used as an alternative electrode material for supercapacitors. Highlights: β-Co(OH)2 nanosheets with hexagonal structure were prepared in NaNO3 solution by using CGDE. The preparation mechanism was discussed based on emission spectra, solution pH and structural characterization. The specific surface area and average pore size of nanosheets are 122.1 m2 g−1 and 33.1 nm, respectively. β-Co(OH)2 nanosheets has 335 F g−1 specific capacitance and 77.1% capacity retention after 3000 cycles. [ABSTRACT FROM AUTHOR]