In this article, a novel Ag₂S nanoparticle-decorated MoS₂ composite (A@M) was synthesized through a facile in situ growth of the monoclinic crystallographic Ag₂S on MoS₂ nanosheets. The A@M composite was used as a catalyst in water splitting which exhibits higher electrocatalytic and photoelectrocatalytic activity than the respective pure MoS₂ and Ag₂S counterparts. Experimental results indicate that the as-prepared A@M composite with an optimal Ag₂S/MoS₂ molar ratio of 16.30% (16%A@M) shows the best catalytic performance with low overpotentials (110 mV for V_oc, 190 mV for onset overpotential, 208 mV for the current density of 20 mA cm⁻²), a small Tafel slope (42 mV dec⁻¹), and a high photocurrent (82 μA cm⁻² under an applied potential of 0.4 V). The enhanced electrocatalytic activity is associated with the improved electrical conductivity resulting from the stretched MoS₂ nanosheets and the enriched active sites due to the decorated Ag₂S particles. The formation of a type II heterojunction structure at the interface between Ag₂S and MoS₂ facilitates the separation of photogenerated charge carriers, and thus is responsible for the enhanced photoelectrocatalytic activity and photocatalytic H₂ production rate (628 μmol h⁻¹ g⁻¹). This work suggests a promising choice to overcome the intrinsic drawbacks of MoS₂ nanostructures for the application in hydrogen evolution.