We present a tunable Q‐band cavity for performing electron paramagnetic resonance and electron nuclear double resonance experiments at cryogenic temperatures. The resonator is a TE011 brass cavity with radio frequency and magnetic field modulation (100 kHz) posts inside the microwave resonator. These posts are located close to the central sample position and thereby enhance the effective modulation and radio frequency fields. The crucial novel design aspect is the provision for continuously tuning the cavity resonant frequency, even under pumped liquid helium conditions over a broad frequency range (≳2.0 GHz). Such a range will compensate for cooling‐induced cavity contraction, presence of cryogenic in the cavity, and insertion of high dielectric frozen aqueous samples. The tuning range is indispensable for use with present commercial Q‐band bridges whose low noise Gunn diode oscillators provide a small ∼50 MHz tuning range. © 1996 American Institute of Physics.