Rational design of advanced cost-effective electrocatalysts is vital for the development of water electrolysis. Herein, we report a novel binder-free efficient Co9S8@Co3O4 core/shell electrocatalysts for oxygen evolution reaction (OER) via a combined hydrothermal-sulfurization method. The sulfurized net-like Co9S8 nanoflakes are strongly anchored on the Co3O4 nanowire core forming self-supported binder-free core/shell electrocatalysts. Positive advantages including larger active surface area of Co9S8 nanoflakes, and reinforced structural stability are achieved in the Co9S8@Co3O4 core/shell arrays. The OER performances of the Co9S8@Co3O4 core/shell arrays are thoroughly tested and enhanced electrocatalytic performance with lower over-potential (260 mV at 20 mA cm−2) and smaller Tafel slopes (56 mV dec−1) as well as long-term durability are demonstrated in alkaline medium. Our proposed core/shell smart design may provide a new way to construct other advanced binder-free electrocatalysts for applications in electrochemical catalysis.