Herein, we report the preparation of interconnected nanosheet assembled microsphere-shaped NiMoO4 and its application in a symmetric aqueous supercapacitor using K3Fe(CN)6 (KFe) as redox-additive. The results show that outstanding capacity of 4648 F g−1 (516 mAh g−1) at 2 A g−1 are recorded in KOH/KFe-0.1 M mixed electrolyte, which is 3.5-fold greater than the value in KOH (1335 F g−1/148 mAh g−1). A high capacitance of 1800 F g−1 (200 mAh g−1) can be achieved at 10 A g−1 with a retention of 73.9% when 0.03 M KFe was added. The stable voltage of the symmetric device assembled by 0.03 M KFe involved electrolyte can be up to 1.5 V, higher than the decomposition voltage of water. Owing to the synergistic reaction of nanostructured NiMoO4 and KFe, the device delivers high energy (41.5 Wh kg−1) and power output (6.8 kW kg−1), outstanding rate capability (87.1%) as well as good cycling performance. These results provide an efficient strategy toward the development of high-performance metal oxide-based symmetric capacitor.