Semiconductor nanocrystals are intriguing because they show surface plasmon absorption features like noble metallic nanoparticles. In contrast with metal, manipulation of their unique plasmonic resonance could be easily realized by the free-carrier concentration. Here, it is demonstrated that MoO_3–x nanodots can exhibit striking surface plasmon resonance located at near-infrared region under treatment of two different reducing agents. Furthermore, the tunable resonance mode has been achieved through appropriate redox processes. Refractive index sensing has been demonstrated by monitoring the plasmonic peak. The improved sensing application is ascribed to the enhanced electric field in the plasmonic nanocrystals. These new insights into MoO_3–x nanodots pave a way to develop novel plasmonic applications such as photothermal therapy, light harvesting, and sensing.