This letter presents the first fabrication and characterization of a biodegradable coaxial cavity resonator, focusing on the measurement of complex permittivity of encapsulation as well as |S₁₁| and impedance parameters. The resonator components are 3D-printed from plant-based resin, coated with silver-coated copper flakes, and enclosed by a laser-cut zinc membrane. A monopole coupler antenna, inspired by the “Great Seal Bug,” is co-designed with the cavity to enable near-field coupling and achieve frequency-selective, near- 50 Ω impedance-matched wireless sensing. Numerical and experimental analysis of the gap between post and membrane (G-post), and between the coupler antenna and post, resulted in| S₁₁ | of −30.3 dB at 1.7 GHz, and a quality factor of 307, outperforming existing flat biodegradable resonators. A 40-MHz resonance shift is observed with a 20 μm variation in G-post, highlighting the resonator’s high sensitivity to membrane position. This system enables battery-free wireless sensing with biodegradable antennas for biodiversity monitoring.