Apparatus and methods for power amplifiers with an injection-locked oscillator driver stage are provided herein. In certain configurations, a multi-mode power amplifier includes a driver stage implemented using an injection-locked oscillator and an output stage having an adjustable supply voltage that changes based on a mode of the multi-mode power amplifier. By implementing the multi-mode power amplifier in this manner, the multi-mode power amplifier exhibits excellent efficiency, including when the voltage level of the adjustable supply voltage is relatively low.