In situ binder-free BiNiPO₄ nanoheterostructures were successfully synthesized through a controlled hydrothermal approach and investigated for multifunctional electrochemical energy-storage and conversion applications. The fabricated BiNiPO₄ electrode exhibited a high specific capacitance of 650 F g⁻¹ at 1 A g⁻¹ with excellent reversibility and rate capability, attributed to its porous hierarchical architecture and strong interfacial interactions. Furthermore, the material demonstrated remarkable bifunctional electrocatalytic activity toward water splitting. For the oxygen evolution reaction (OER) in 1 M KOH, the electrode achieved a low overpotential of 260 mV at 50 mA cm⁻² with a Tafel slope of 59 mV dec⁻¹, while for the hydrogen evolution reaction (HER) in 1 M H₂SO₄, it delivered an overpotential of 160 mV at 50 mA cm⁻² and a Tafel slope of 70 mV dec⁻¹. The enhanced electrochemical performance is mainly attributed to the synergistic redox coupling, large electrochemically active surface area, rapid charge- transfer kinetics, efficient ion diffusion, and robust structural stability provided by the hierarchical heterointerface. These findings highlight BiNiPO₄ as a promising binder-free bifunctional electrode material for advanced supercapacitor and water-electrolysis technologies.