Abstract Two‐dimensional (2D) semiconductors have recently emerged as a remarkable class of plasmonic alternative to conventional noble metals. However, tuning of their plasmonic resonances towards different wavelengths in the visible‐light region with physical or chemical methods still remains challenging. In this work, we design a simple room‐temperature chemical reaction route to synthesize amorphous molybdenum oxide (MoO 3− x ) nanodots that exhibit strong localized surface plasmon resonances (LSPR) in the visible and near‐infrared region. Moreover, tunable plasmon resonances can be achieved in a wide range with the changing surrounding solvent, and accordingly the photoelectrocatalytic activity can be optimized with the varying LSPR peaks. This work boosts the light–matter interaction at the nanoscale and could enable photodetectors, sensors, and photovoltaic devices in the future.