We demonstrate polyimide (PI) nonwoven fabric-reinforced, flexible proton-conductive phosphosilicate glass composite membranes for potential application in high-temperature/low-humidity proton exchange membrane fuel cells (PEMFCs). The new reinforced composite membrane is fabricated via the impregnation of a 3-glycidyloxypropyl trimethoxysilane (GPTMS)/orthophosphoric acid (H3PO4) mixture into a PI nonwoven substrate followed by in situ sol–gel synthesis and hydrothermal treatment. This unique structural integrity enables the reinforced composite membrane to provide unprecedented improvement in the mechanical properties (notably flexibility and thickness) over typical bulk phosphosilicate glasses that are highly fragile and thick. Meanwhile, the highly porous structure of the PI reinforcing framework allows for the facile formation of a three-dimensionally interconnected phosphosilicate glass matrix in the reinforced composite membrane, which in turn offers favorable pathways for proton transport. Another advantageous feature of the reinforced composite membrane is higher proton conductivity under dehumidified conditions, as compared to a hydration-dependent polymer electrolyte membrane such as sulfonated poly(arylene ether sulfone) (SPAES). This superior proton conductivity of the reinforced composite membrane is further discussed with in-depth consideration of its architectural novelty and proton transport phenomena governed by the Grotthuss mechanism.