The durability of a dye-sensitized solar cell with flexible substrate (fDSSC) was improved by introducing a composite separator membrane (CSM) between the TiO2 working electrode and the counter electrode (CE). The fDSSC was fabricated by replacing a typical fluorine-doped tin oxide glass substrate of the platinum CE with a flexible indium tin oxide-coated poly(ethylene naphthalate) substrate. The CSM was a triple-layered membrane composed of poly(ethylene terephthalate) nonwoven fabric between the poly(vinylidene fluoride-co-hexafluoropropylene) gel layers, which provided a sandwich-like structure. The fDSSC with the CSM exhibited relatively good electrolyte retention capability and improved interfacial contact between the components, affording an fDSSC with high energy conversion efficiency and stable long-term performance. The potential of a CSM as an efficient component for durable flexible energy devices is demonstrated.