The electrocatalytic water splitting activity of layered transition metal dichalcogenides (TMDs) is limited by inert basal planes and slower reaction kinetics. Here, we demonstrate the use of MoSe2–Cu2S nanoheterostructures (NHSs) as an efficient electrocatalyst for water splitting. The interfacial engineering of the basal planes of MoSe2 nanosheets (NSs) with Cu2S nanocrystals (NCs) increases the number of active sites for oxygen evolution reaction (OER), owing to the decoration of inert basal planes of MoSe2 NSs with an OER active material i.e. Cu2S NCs. Density functional theory calculations revealed a favorable type-II band alignment, and built-in electric field at heterointerfaces, significantly decreasing the energy barrier for interfacial charge transfer. As a consequence of the simultaneous increase of number of active sites and modulation of the electronic structure, the MoSe2–Cu2S NHSs exhibit higher OER activity with an overpotential of 264 mV at 10 mA cm−2 current density. The built-in electric field phenomenon also greatly promotes hydrogen evolution reaction (HER) activity of MoSe2–Cu2S NHSs in alkaline medium on account of the decreased kinetic energy barrier. This study sheds light on the importance of designing NHS-derived systems to tune the electrocatalytic properties for water splitting.