Surface plasmon resonance (SPR) sensors based on a silver film suffer from signal degradation due to silver oxidation in aqueous sensing environments. To overcome this limitation, we fabricated the planar plasmonic substrate employing an atomic MoS2 layer on a silver surface. Successful production of a large-area MoS2 monolayer blocks the penetration of oxygen and water molecules. In addition, we theoretically and experimentally found that MoS2 layer on the silver film can improve the SPR sensitivity and stability significantly. In this study, the proposed SPR substrate has the potential to provide highly enhanced sensor platforms for surface-limited molecular detections.