It is of essential importance to design an electrocatalyst with excellent performance for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in water splitting. Co₃O₄ has been developed as a highly efficient OER electrocatalyst, but it has almost no activity for HER. In a previous study, it has been demonstrated that the formation of oxygen vacancies (V_O) in Co₃O₄ can significantly enhance the OER activity. However, the stability of V_O needs to be considered, especially under the highly oxidizing conditions of the OER process. It is a big challenge to stabilize the V_O in Co₃O₄ while reserving the excellent activity. Filling the oxygen vacancies with heteroatoms in the V_O-rich Co₃O₄ may be a smart strategy to stabilize the V_O by compensating the coordination numbers and obtain an even surprising activity due to the modification of electronic properties by heteroatoms. Herein, we successfully transformed V_O-rich Co₃O₄ into an HER-OER electrocatalyst by filling the in situ formed V_O in Co₃O₄ with phosphorus (P-Co₃O₄) by treating Co₃O₄ with Ar plasma in the presence of a P precursor. The relatively lower coordination numbers in V_O-Co₃O₄ than those in pristine Co₃O₄ were evidenced by X-ray adsorption spectroscopy, indicating that the oxygen vacancies were created after Ar plasma etching. On the other hand, the relatively higher coordination numbers in P-Co₃O₄ than those in V_O-Co₃O₄ and nearly same coordination number as that in pristine Co₃O₄ strongly suggest the efficient filling of P in the vacancies by treatment with Ar plasma in the presence of a P precursor. The Co–O bonds in Co₃O₄ consist of octahedral Co³⁺(O_h)–O and tetrahedral Co²⁺(T_d)–O (Oh, octahedral coordination by six oxygen atoms and T_d, tetrahedral coordination by four oxygen atoms). More Co³⁺(O_h)–O are broken when oxygen vacancies are formed in V_O-Co₃O₄, and more electrons enter the octahedral Co 3d orbital than those entering the tetrahedral Co 3d orbital. Then, with the filling of P in the vacancy site, electrons are transferred out of the Co 3d states, and more Co²⁺(T_d) than Co³⁺(O_h) are left in P-Co₃O₄. These results suggest that the favored catalytic ability of P-Co₃O₄ is dominated by the Co²⁺(T_d) site. P-Co₃O₄ shows superior electrocatalytic activities for HER and OER (among the best non-precious metal catalysts). Owing to its superior efficiency, P-Co₃O₄ can directly catalyze overall water splitting with excellent performance. The theoretical calculations illustrated that the improved electrical conductivity and intermediate binding by P-filling contributed significantly to the enhanced HER and OER activity of Co₃O₄.