Bifunctional catalysts that are highly active toward both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are attractive for efficient electrochemical water splitting. Herein, we report a bifunctional FeCoOOH nanosheet catalyst for highly efficient electrochemical water splitting in an alkaline electrolyte. The FeCoOOH nanosheet arrays were grown directly on the surface of a porous Ni foam by using a simple hydrothermal method. Because of their binary oxyhydroxide structure and high electrical conductivity intrinsic to direct growth, these FeCoOOH nanosheets exhibited excellent activities toward both the HER and OER. With the use of this bifunctional FeCoOOH catalyst, an alkaline water electrolyzer in a two-electrode configuration achieved 10 mA cm^-2 only at a cell voltage of 1.62 V without iR compensation in 1M KOH, which outperformed that based on the combination of commercial IrO₂ and Pt/C catalysts.