In this work, niobium diboride (NbB2) nanoparticles were successfully synthesized using a simple and facile molten salt method and examined as electrodes for supercapacitors. The synthesized nanoparticles were characterized using SEM, HRTEM, XRD, XPS, and BET techniques. Also, the electrochemical performance of various NbB2 nanoparticles depending on the various amorphous boron concentrations was systematically investigated using both a neutral (1 M Na2SO4) and a basic (6 M KOH) electrolyte. Electrochemical analysis revealed that the obtained sample denoted as the NbB2–8 exhibited the best electrochemical performance (159.6 F/g at 10 mV/s) in Na2SO4 electrolyte compared to that of the KOH electrolyte (69.6 F/g at 10 mV/s). Such a result can be owing to the size and mobility of ions in the electrolyte. Overall, this work could pave the way for other metal borides’ versatility and potential in energy storage.