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2D materials are regarded as promising electrocatalysts for water splitting because of their advances in providing ample active sites and improving electrochemical reaction kinetics. 2D MoSe2 has a greater intrinsic electrical conductivity and lower Gibbs free energy for reactant adsorption. However, there is still room for improvement in the electrocatalytic performance of MoSe2 for high‐performance electrochemical water splitting devices. Herein, the in situ preparation of heterostructure made of covalently bonded MoSe2 and rGO is reported. The obtained electrocatalyst contains the aggregated 3D structured MoSe2 over rGO, which is covalently bonded together with more edge sites. The active edge sites of MoSe2/rGO are dynamically involved in the electrocatalytic activity while facilitating electron transfer. Hence, the MoSe2/rGO heterostructure requires a low cell voltage of 1.64 V to reach 100 mA cm−2 in water splitting with high reaction kinetics. The aggregated MoSe2 over rGO with more edge sites exposed by the 3D structure of MoSe2 and the interfacial covalent bond in between them provides a favorable electronic structure for the HER and OER with low overpotentials and high current densities and enhances the stability of the electrocatalyst. This work presents an attractive and cost‐effective electrocatalyst suitable for industrial‐scale hydrogen fuel production. [ABSTRACT FROM AUTHOR]