A cold-plasma-based technique for tuning an evanescent-mode cavity resonator is introduced and studied experimentally for the first time in this paper. The technique involves a plasma jet that constitutes a variable resistance integrated in the cavity. The electron density and consequently the electromagnetic properties of plasma, including its resistivity, are controlled by varying the magnitude of the sinusoidal excitation voltage. The transmission coefficient of the two-port fabricated resonator at 2.735 GHz exhibits 11 dB tunability when the magnitude of the 20-kHz plasma-excitation voltage increases from zero to 5.26 kV (peak-to-peak). The resonator's quality factor varies in the acceptable range of 684-342 for these conditions. The measured and simulated results reveal that this approach may become a promising tuning technology particularly in demanding applications where conventional solid-state techniques are ineffective due to temperature, power, or linearity limitations.