Efficient and durable oxygen evolution reaction (OER) catalysts are highly required for the cost‐effective generation of clean energy from water splitting. For the first time, an integrated OER electrode based on one‐step direct growth of metallic iron–nickel sulfide nanosheets on FeNi alloy foils (denoted as FeNi3S2/FeNi) is reported, and the origin of the enhanced OER activity is uncovered in combination with theoretical and experimental studies. The obtained FeNi3S2/FeNi electrode exhibits highly catalytic activity and long‐term stability toward OER in strong alkaline solution, with a low overpotential of 282 mV at 10 mA cm−2 and a small Tafel slope of 54 mV dec−1. The excellent activity and satisfactory stability suggest that the as‐made electrode provides an attractive alternative to noble metal‐based catalysts. Combined with density functional theory calculations, exceptional OER performance of FeNi3S2/FeNi results from a combination of efficient electron transfer properties, more active sites, the suitable O2 evolution kinetics and energetics benefited from Fe doping. This work not only simply constructs an excellent electrode for water oxidation, but also provides a deep understanding of the underlying nature of the enhanced OER performance, which may serve as a guide to develop highly effective and integrated OER electrodes for water splitting. Self‐supported oxygen evolution reaction (OER) electrodes consisting of iron–nickel sulfide nanosheets on FeNi foils are synthesized by a one‐step in situ synthetic method, where the FeNi foil works as the sources of Fe, Ni, and the current collectors, simultaneously. The as‐made integrated electrode shows excellent electrocatalytic activity and long term stability toward water oxidation in alkaline conditions and provides an attractive alternate to noble metal‐based catalysts. Furthermore, the origin of the enhanced OER activity is uncovered using density functional theory calculations in this work.