 Polyaniline–nickel oxide (PANI-NiO) nanocomposites were synthesized by in situ aqueous oxidative polymerization and were used for supercapacitor applications. The NiO nanosheets were prepared using sodium dodecyl sulfate as the anionic surfactant. The morphology and microstructure of the samples were examined by X-ray diffraction, scanning electron microscope, and thermogravimetric analysis. The obtained results indicated that the NiO nanosheets were uniformly distributed in the polyaniline (PANI) nanofibrous matrix. The electrochemical properties of the PANI-NiO nanocomposites were studied using cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy. Notably, such nanocomposites showed high specific capacitances up to 514 F g−1 at a scan rate of 1 mV s−1.