A high sensitivity tunable power detector is proposed in the paper. An input matching network is necessary in a power detector to obtain suitable input reflection coefficient and reasonable sensitivity. A resistive- or reactive-type network is usually employed for wideband or high sensitivity purpose, respectively. However, it implies a trade-off between bandwidth and sensitivity. In order to simultaneously achieve high sensitivity and reflection coefficient, a tunable matching network, composed of a single hyperabrupt junction varactor, is investigated to realize the tunable power detector. The measurements show that the S11 below −10 dB is from 1.36 GHz to 3.36 GHz and the voltage sensitivity at 1.4 GHz, 2.4 GHz and 3.3 GHz is 903 mV/mW, 3579 mV/mW and 2646 mV/mW, respectively, with the corresponding tuning voltage.