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Exploring bifunctional electrocatalysts with high efficiency, inexpensive, and easy integration is still the daunt challenge for the production of hydrogen on a large scale by means of water electrolysis. In this work, a novel free-standing Co 3 S 4 /CoMo 2 S 4 heterostructure on nickel foam by a facial hydrothermal method is demonstrated to be an effective bifunctional electrocatalyst for overall water splitting (OWS). The synthesized Co 3 S 4 /CoMo 2 S 4 electrocatalyst achieves ultralow overpotentials of 143 mV@10 mA cm −2 for hydrogen evolution reaction (HER) and 221 mV@25 mA cm −2 for oxygen evolution reaction (OER), respectively, in 1 M KOH. Moreover, it presents a greatly improved durability and stability under operando electrochemical conditions. When used as catalysts for OWS, the Co 3 S 4 /CoMo 2 S 4 -3//Co 3 S 4 /CoMo 2 S 4 -3 electrodes just need 1.514 V to make it to the current density of 10 mA cm −2 . It is supposed that the introduction of heterogeneous interface between Co 3 S 4 and CoMo 2 S 4 could give rise to plentiful active sites and enhanced conductivity, and thus boost excellent catalytic performances. Moreover, the porous feature of free-standing nanosheets on nickel foam could benefits catalytic performances by accelerating charge transport and releasing bubbles rapidly. This work proposes a bifunctional catalyst system with the heterogeneous interface, which could be used in a sustainable green energy system.