The purpose of this study is to develop novel poly(vinyl alcohol)–2-acrylamido-2-methyl-1-propanesulfonic acid (PVA–PAMPS) blend membranes as suitable alternative materials to perfluorinated ionomers for application in polymer electrolyte fuel cells. The membrane structure is finished by cross-linking the hydroxyl groups of poly(vinyl alcohol) (PVA) main chain with acetal ring formation using glutaraldehyde cross-linker. The flexible side chains are further introduced with octylaldehyde as a hydrophobicizer through an acetal, which is obtained via reaction of PVA with octylaldehyde. PVA–PAMPS blend membranes are designed to possess all the required properties of a proton exchange membrane, namely, reasonable swelling, good mechanical strength and flexibility, low methanol permeability, along with the high proton conductivity due to “trapped” PAMPS chains in the PVA network. The membranes show an isotropic swelling and a large sorption of non-freezing water relative to the freezing water. High proton conductivities (0.093–0.12 S cm⁻¹ at 25 ± 2 °C) are obtained for a polymer composition PVA–PAMPS being 1 : 2 in mass. Despite their high ion exchange capacities (1.71–1.91 meq g⁻¹) and high water uptakes (1.36–1.63 g g⁻¹ dry polymer), the membranes display good mechanical properties (tensile strength: 1.3–2.2 kg mm⁻² and elongation: 7.2–16.2%). The membranes exhibit an excellent stability toward water (e.g., 25 °C for 240 h and 50 °C for 100 h) for a long time without any decrease in proton conductivity and the reproducibility of proton conductivity (20–50 °C), which suggests a good lifetime usage of the membranes.