An RF-MEMs based tri-band GaN power amplifier with state-of-the-art performance is presented. The active device is a low-cost GaN-on-Si transistor with a plastic package. A dedicated broadband output matching network is designed to provide the desired waveform at the device intrinsic level. The frequency reconfigurability is implemented with one RF-MEMS at the input stage. Measured results show a 45, 28 and 46% power added efficiency (PAE) for an output power of 5.6, 3.0 and 4.5 W at 1.4, 2.5 and 3.6 GHz, respectively. Furthermore, the power amplifier exhibits a high drain efficiency of more than 46% across 1.3–1.4 and 3.4–3.7 GHz whilst delivering an output power of more than 4.3 W.