Tunable plasmon resonances in suspended 2D molybdenum oxide flakes are demonstrated. The 2D configuration generates a large depolarization factor and the presence of ultra-doping produces visible-light plasmon resonances. The ultra-doping process is conducted by reducing the semiconducting 2D MoO3 flakes using simulated solar irradiation. The generated plasmon resonances can be controlled by the doping levels and the flakes' lateral dimensions, as well as by exposure to a model protein.