A thin-film composite (TFC) membrane for ethanol dehydration by pervaporation with a very high flux and a moderately high separation factor was fabricated using a strong polyelectrolyte hydrogel as an active layer. The thin-film hydrogel was graft-polymerized with vinyl sulfonic acid (VSA) as the polymer monomer and N, N′-methylenbisacrylamide (MBAA) as the cross-linker monomer, employing the UV photo-initiation method on a polyethersulfone ultrafiltration support. The successful grafting was confirmed by ATR-FTIR, and AFM and SEM studies showed that TFC membranes with an active layer of 100–300 nm were fabricated. The IEC of the active layer increased up to ca. 3 meq/g when the cross-linker fraction was changed from 1% to 5% MBAA and then decreased at 10% MBAA, probably due to the high cross-linker fraction in the active layer. The degree of swelling decreased with the cross-linker concentration and with the duration of the UV irradiation. The pervaporation performance of the TFC membranes was studied with a 90% ethanol solution at 50 °C. The separation factor coincided with the increase in the IEC, which is attributed to the higher sorption of water molecules to the active layer. The degree of swelling had a lesser influence on the membrane performance than the IEC, but greater swelling decreased the performance for similar IEC values. The optimal membrane with a 7.5 kg m−2 h−1 flux and a separation factor of 313 was obtained at the highest IEC and a limited degree of swelling, making it one of the membranes with the highest pervaporation separation index reported to date. However, the stability of the membrane should be improved, as its high performance decreased after 24–48 h and reached a plateau after a few days of operation.