A controllable approach that combines surface plasmon resonance and two-dimensional (2D) graphene/MoS2 heterojunction has not been imple?mented despite its potential for efficient photoelectrochemical (PEC) water splitting. In this study, plasmonic Ag-decorated 2D -MoS2 nanosheets were vertically grown on graphene substrates in a prac?tical large-scale manner through metalorganic chemical vapor depo?sition of -MoS2 and thermal evapo?ration of Ag. The plasmonic Ag-decorated -MoS2 nanosheets on graphene yielded up to 10 times higher photo-to-dark current ratio than -MoS2 nanosheets on indium tin oxide. The significantly enhanced PEC activity could be attributed to the synergetic effects of SPR and favorable graphene/2D -MoS2 heterojunction. Plasmonic Ag nanoparticles not only increased visible-light and near-infrared absorption of 2D -MoS2, but also induced highly amplified local electric field intensity in 2D -MoS2. In addition, the vertically aligned 2D -MoS2 on graphene acted as a desirable heterostructure for efficient separation and transportation of photo-generated carriers. This study provides a promising path for exploiting the full potential of 2D -MoS2 for practical large-scale and efficient PEC water-splitting applications.