This letter presents a resonant tunneling diode (RTD)-based microwave amplifier operating at deep sub-milliwatt level dc-power. The fabricated amplifier, which is based on a reflection-type amplifying topology and uses an InP monolithic microwave integrated circuit technology, shows extremely low dc-power consumption of 125 mu W with a gain of more than 10 dB at 5.7 GHz. The amplifier performance is mainly enabled by the favorable characteristics of the 0.9 - mu m InP-based RTDs biased at V-BIAS = 0.36V. The RTDs exhibit a high peak-to-valley current ratio (PVCR) of 11.2 with a low peak current (I-P) of 430 mu A and thereby a relatively low negative resistance magnitude of 480 Omega. The dc-power consumption is about 6.4 times lower than that in transistor-based low-power amplifiers reported to date for the 5 GHz frequency band.