Hard carbon has attracted enormous attentions as electrode materials for various energy storage devices owing to its low cost, abundant sources and high theoretical capacitances/capacities. However, it still suffers poor electrical conductivity and unsatisfying rate performance. Herein, N/S co-doped ultrathin carbon nanosheet is reported as high performance electrode material for both supercapacitors and sodium-ion batteries. The symmetric supercapcitors assembled with the as-synthesized materials (1.0 M Na2SO4 as electrolyte) deliver a high energy density of 24.5 Wh kg−1 at a power density of 500 W kg−1 within a wide voltage range of 0–2.0 V. When being served as anode for sodium-ion batteries, the electrode exhibits a high reversible capacity (371.2 mAh g−1 @ 0.1 A g−1) and superior rate capability (180.9 mAh g−1 @ 10.0 A g−1). Besides, the as-fabricated electrodes display stable cycling performance in both energy storage systems. Subsequent mechanism studies further shed light on the excellent electrochemical performance.