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As bendable, conformal electronic components, flexible gigahertz resonators are in demand as important building blocks (e.g., oscillators, filters, and signal processors) in future flexible radio‐frequency electronics for efficient wireless communication. Here, a 2.7 GHz piezoelectric thin‐film resonator (i.e., film bulk acoustic wave resonator) is presented that possesses high levels of both electrical performance and mechanical flexibility. The highly bendable resonator fabricated by FlexMEMS technology is essentially a thin‐film composite (i.e., high‐quality metal and inorganic piezoelectric layer stacks) encapsulated in polymer thin films with a total thickness of only 11.6 µm. The experimental series resonance frequency (fS), parallel resonance frequency (fP), quality factor (Q), and effective coupling coefficient ( kt  eff 2) are 2.72 GHz, 2.77 GHz, 1398, and 4.39%, respectively. These parameter values are comparable to those of a conventional silicon‐based resonator. The minimum bending radius of the flexible resonator can be reduced to ≈0.5 mm with very slight electrical performance variation. Furthermore, the flexible resonator retains its mechanical and electrical stability after 2000 bending cycles. The superior mechanical flexibility and stability represent a significant advancement toward bendable, foldable, and conformal electronics working in the RF range. A highly flexible gigahertz piezoelectric resonator shows ultrahigh mechanical bendability when encapsulated in a polymer thin film package with a total thickness of only 11.6 µm. The flexible device possesses comparable performance to conventional resonators on a silicon substrate and remains fully functional even when bent into a radius of 0.5 mm and after 2000 bending cycles. [ABSTRACT FROM AUTHOR]