We have demonstrated high-performance flexible germanium (Ge) vertical p−i−n photodetectors (PDs) based on a resonant cavity structure by a direct flip transfer of Ge nanomembranes on polyethylene terephthalate (PET) substrates. Finite-difference time-domain simulation proves that the vertical cavity structure composed of the bottom gold and top SU-8 layers as a reflector and an anti-reflection surface, respectively, could enhance the average absorption in the near-infrared (NIR) region (i.e., 1520−1640 nm) from 0.06 to 0.20 by 233%. Strains introduced into Ge NMs by convex and concave fixtures are measured to be 0.37 and −0.32%, respectively. The fabricated PDs exhibit a low dark current density of 9.6 mA/cm 2 at −1 V and a high forward−reverse current ratio of 10 5 under the flat condition. Responsivity at 1550 nm increases from 52.5 to 133.8 mA/W by tensile strain, while it slightly decreases to 32.6 mA/W under comparable compressive strain. Furthermore, the devices show no degradation in their optoelectronic responses after 200 bending cycles at convex fixtures with a radius of 30 mm. Overall, such flexible Ge PDs with the capabilities of both excellent optoelectronic performance and mechanical durability represent significant advances in the field of group IV NIR optoelectronic devices.