A novel thermosensitive interpenetrating network (IPN) hydrogel was prepared from an aqueous solution of poly[3,3-dimethyl(methacryloyloxyethyl)-propylammonium sulfonate] infiltrating a crosslinked sulfobetaine polymer, poly(2-acrylamido-2-methylpropylsulfonic acid). The IPN gel shows an upper critical solution temperature (UCST) depending on the molar ratio of the components and the presence of salt. In contrast to conventional gels, the IPN gel exhibits no volume phase transition after traversing the UCST.