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• Mesoporous CoS x is self-grown on the Co film by a cheap anodization method. • CoS x is an active bifunctional electrocatalyst towards overall water splitting. • Amorphous CoS x shows a better electrochemical performance than crystallized phase. Electrocatalysts based on transition metal sulfides (TMSs) are drawing accelerating concerns in renewable energy research because of their intrinsically excellent activities towards both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). To date, considerable efforts are made to improve the performance of these catalysts, but ignoring the improper synthesis strategy would incur additional cost to the catalyst. Herein, a convenient, one-step anodization method is developed for fast construction of cobalt sulfides (CoS x). Without any high-temperature or long-time treatment, mesoporous CoS x is self-grown on the metal substrate in minutes. As a result, as-anodic CoS x requires overpotentials of 102 mV for HER and 284 mV for OER to achieve a current density of 10 mA m−2 in alkaline solution. Moreover, the tandem bifunctional as-anodic CoS x exhibits a required cell voltage of 1.64 V for overall water splitting in alkaline solution, exceeding most of the documented Co-based electrocatalysts. Mesoporous cobalt sulfides (CoS x) were self-grown on the metal substrate via a facile, one-step anodization method for high efficient overall water splitting. [Display omitted] [ABSTRACT FROM AUTHOR]