Highly active, low-cost, and durable electrocatalysts for the water oxidation reaction are pivotal in energy conversion and storage schemes. Here we report a nitride-core, oxide-shell-armor structured FeCoNi oxynitride as an efficient oxygen evolution electrocatalyst with a homogeneous nitride (Fe0.70Co0.56Ni0.92N1.0O0.06) core and an oxide (Fe0.48Co0.1Ni0.21N0.05O1.0) shell. The catalyst demonstrated excellent activity for the oxygen evolution reaction with a current density of 10 mA cm(-2) at a low overpotential of 0.291 V in alkaline media (1 M KOH), which is superior to the activities of commercial IrO2, RuO2, and Pt/C catalysts and comparable to those of state-of-the-art catalysts (e.g., NiFe-LDH, NiCo2O4, O-NiCoFe-LDH). Density functional theory simulations suggested that the incorporation of multiple metal elements can indeed improve the reaction energetics with a synergistic effect from the core-shell structure. This unique structure of a nitride-core with a oxide-shell presents a new form of multimetallic oxynitride with compelling performance in electrolytic oxygen evolution.