Hybrid organic/inorganic membranes that include a functionalized (-SO3H), interconnected silica network, a non-porogenic organic matrix, and a -SO3H-functionalized terpolymer are synthesized through a sol–gel-based strategy. The use of a novel crosslinkable poly(vinylidene fluoride-ter-perfluoro(4-methyl-3,6-dioxaoct-7-ene sulfonyl fluoride)-ter-vinyltriethoxysilane) (poly(VDF-ter-PFSVE-ter-VTEOS)) terpolymer allows a multiple tuning of the different interfaces to produce original hybrid membranes with improved properties. The synthesized terpolymer and the composite membranes are characterized, and the proton conductivity of a hybrid membrane in the absence of the terpolymer is promising, since 8 mS cm−1 is reached at room temperature, immersed in water, with an experimental ion-exchange-capacity (IECexp) value of 0.4 meq g−1. Furthermore, when the composite membranes contain the interfaced terpolymer, they exhibit both a higher proton conductivity (43 mS cm−1 at 65 °C under 100% relative humidity) and better stability than the standard hybrid membrane, arising from the occurrence of a better interface between the inorganic silica and the poly[(vinylidene fluoride)-co-hexafluoropropylene] (poly(VDF-co-HFP)) copolymer network. Accordingly, the hybrid SiO2-SO3H/terpolymer/poly(VDF-co-HFP) copolymer membrane has potential use as an electrolyte in a polymer-electrolyte-membrane fuel cell operating at intermediate temperatures.