This paper reports the fabrication of β-Ga₂O₃ nanomembrane (NM) based flexible photodetectors (PDs) and the investigation of their optoelectrical properties under bending conditions. Flexible β-Ga₂O₃ NM PDs exhibited reliable solar-blind photo-detection under bending conditions. Interestingly, a slight shifting in wavelength of the maximum solar-blind photo-current was observed under the bending condition. To investigate the reason for this peak shifting, the optical properties of β-Ga₂O₃ NMs under different strain conditions were measured, which revealed changes in the refractive index, extinction coefficient and bandgap of strained β-Ga₂O₃ NMs due to the presence of nano-sized cracks in the β-Ga₂O₃ NMs. The results of a multiphysics simulation and a density-functional theory calculation for strained β-Ga₂O₃ NMs showed that the conduction band minimum and the valence band maximum states were shifted nearly linearly with the applied uniaxial strain, which caused changes in the optical properties of the β-Ga₂O₃ NM. We also found that nano-gaps in the β-Ga₂O₃ NM play a crucial role in enhancing the photoresponsivity of the β-Ga₂O₃ NM PD under bending conditions due to the secondary light absorption caused by reflected light from the nano-gap surfaces. Therefore, this research provides a viable route to realize high-performance flexible photodetectors, which are one of the indispensable components in future flexible sensor systems.