<p>The development of high-performance, durable electrode materials is paramount for advancing supercapacitor technology. While magnetite (Fe₃O₄) offers high theoretical pseudocapacitance, its practical application is hindered by intrinsic poor conductivity and structural instability. Here, we demonstrate that controlled lanthanum (La³⁺) doping is a highly effective strategy for engineering defects and enhancing the electrochemical properties of Fe₃O₄. Through a facile hydrothermal synthesis, we fabricated pure and La-doped Fe₃O₄ (La–Fe₃O₄) nanostructures with doping concentrations of 1–3%. Comprehensive characterization reveals that La³⁺ incorporation induces lattice strain, reduces crystallite size, and generates oxygen vacancies, thereby optimizing the electronic structure and ion diffusion pathways. Electrochemically, the optimized 3% La–Fe₃O₄ electrode delivers a specific capacitance of 549&#xa0;F g^− 1 at 0.5&#xa0;A g^− 1, significantly outperforming pure Fe₃O₄ (170&#xa0;F g^− 1). Furthermore, a symmetric supercapacitor fabricated with the 3% La–Fe₃O₄ electrode achieves a specific capacitance of 457&#xa0;F g^− 1, an energy density of 21.8 Wh kg^− 1, and cycling stability with 74.8% capacitance retention and 83.2% Coulombic efficiency over 10,000 cycles. This work elucidates the critical role of rare-earth doping in defect engineering and provides a compelling pathway for designing robust, high-energy-density supercapacitor electrodes.</p>