For sustainable hydrogen energy generation systems, safe, repeatable, and cost-effective hydrogen gas-leakage monitoring is required. In this study, transparent and flexible hydrogen gas sensor membranes that satisfy the above requirements were prepared. Pt/WO3-thin films were deposited on the surface of polydimethyl siloxane (PDMS) membranes previously modified with hydroxyethyl acrylamide (HEAAm) via ultraviolet light-induced graft polymerization. Such grafted HEAAm provided an acid- and heat-resistant hydrophilic surface, where hydrogen gasochromic Pt/WO3 particles can be deposited to form a thin film. The thus prepared Pt/WO3 thin film-coated PDMS membranes show significant colorimetric response toward hydrogen gas.