Single-atom catalysts (SACs) have attracted enormous attentions in heterogeneous catalysts due to the maximized atomic utilization and extraordinary catalytic performance. Similar to homogeneous catalytic ligands, the support in SACs plays a vital role in the catalytic properties. Herein, we present a series of transition-metal atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Os, Ir and Pt) anchored on a vanadium diselenide (VSe2) monolayer as electrocatalysts through density functional theory calculations. Pd@VSe2 stands out among the considered SACs with a low overpotential of 0.38 V, exhibiting the excellent performance of oxygen reduction reaction (ORR). Meanwhile, a liner trend between the adsorption Gibbs free energy of the OH (Delta G(OH)*) and the predicted eta(ORR) is revealed, which may serve as a simple descriptor for the inherent ORR catalytic activity of SACs. Particularly, Pt@VSe2 shows extraordinarily low theoretical overpotential of -0.04/0.47 V for hydrogen/oxygen evolution reaction, which transcends the state-of-the-art Pt and IrO2 and thereby can be exploited as highly-efficient bifunctional electrocatalyst for overall water splitting. This work broadens the perception of designing multifunctional electrocatalysts based on two-dimensional VSe2 material and offers a new paradigm for investigating advanced SACs. (C) 2021 Elsevier Inc. All rights reserved.