In this study, a super acid with impressive hygroscopicity, 1-[3-(methacryloyloxy)propylsulfonyl]-1-(triﬂuoromethanesulfonyl)imide (MPTI), is exploited to improve the proton conductivity of PEMs at low humidity. Importantly, MPTI can deliquesce into an aqueous solution by capturing moisture from air at a considerable rate. Investigation of the hygroscopicity of MPTI and the corresponding mechanism by molecular dynamics simulation show a total interaction energy between MPTI and water of −368.13 kJ mol−1, which greatly exceeds those of model derivatives with other typical hygroscopic groups. To apply MPTI in PEMs and prevent leakage, MPTI is incorporated into a semi-interpenetrating polymer network via in situ polymerization, and Nafion-based composite membranes are fabricated. The water uptake of the obtained hybrid membranes is substantially increased by up to 66.61% at 40% RH and 90.04% at 95% RH. This optimization of the water environment facilitates the dissociation of protons and the formation of hydrogen bond networks for high-speed proton conduction. As a result, the proton conductivity of the membranes increases by up to two orders of magnitude at low humidity. Notably, this composite membrane enhanced the performance of a single fuel cell at 60% RH by 41.9%.