Here, we report for the first time a facile ultrasonic synthesis of Fe₃O₄ nanoparticles using FeCl₃ and the organic solvent ethanolamine (ETA). The intermediate of the ETA–Fe(II) complex produces Fe₃O₄ after hydrolysis and hydrothermal treatment. The moderate reduction of ETA and ultrasound play an important role in the synthesis of superfine Fe₃O₄ particles with a very high specific surface area (165.05 m² g⁻¹). The Fe₃O₄ nanoparticles were characterized by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), high-resolution transmission electron microscopy (HRTEM), and ultraviolet-visible absorption spectroscopy (UV-vis). Fe₃O₄ as an electrode material was fabricated into a supercapacitor and characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge measurements. The as-synthesized Fe₃O₄ exhibits remarkable pseudocapacitive activities including high specific capacitance (207.7 F g⁻¹ at 0.4 A g⁻¹), good rate capability (90.4 F g⁻¹ at 10 A g⁻¹), and excellent cycling stability (retention 100% after 2000 cycles). This novel synthetic route towards Fe₃O₄ is a convenient and potential way of producing a secondary energy material which is expected to be applicable in the synthesis of other metal oxide nanoparticles.