The high overpotential of the oxygen evolution reaction (OER) is the main obstacle to water electrolysis for green hydrogen production. NiFe-based materials are the potential non-precious metal electrocatalysts for the OER, but their poor stability and limited activity have been of concern. Herein, a three-dimensional self-supporting NiFe electrode is prepared by direct writing 3D printing using spherical powders of Ni and Fe metals followed by hydrothermal treatment at 130 °C in urea and NH4F mixed solution. The fabricated 3D micro–nano-structured NiFe electrode presents an excellent OER activity of 220 mV overpotential to reach 100 mA cm–2 with a Tafel slope of 49.1 mV dec–1 and stability up to 100 h in continuous or intermittent power supply. The hydrothermal treatment produces in situ growth of flower-like clusters of NiFe2O4 nanoneedles wrapped with FeOOH nanosheets on the surface of the 3D NiFe bimetal microsphere network, and the outstanding OER activity is attributed to a wide variety of interfaces and heterojunctions among metal microspheres, metal oxides, and hydroxides. This study provides an approach for preparing high-performance self-supporting electrodes for electrochemical processes.