Proteins of thermophiles are thermally stable in a high-temperature environment, adopting a strategy of enhancing the electrostatic interaction in hydrophobic media at high temperature. Herein, inspired by the molecular mechanism of thermally stable proteins, the synthesis of novel polymer materials that undergo ultrarapid, isochoric, and reversible switching from soft hydrogels to rigid plastics at elevated temperature is reported. The materials are developed from versatile, inexpensive, and nontoxic poly(acrylic acid) hydrogels containing calcium acetate. By the cooperative effects of hydrophobic interaction and ionic interaction, the hydrogels undergo significant spinodal decomposition and subsequent rubbery-to-glassy transition when heated to an elevated temperature. As a result, the gels exhibit super-rapid and significant hikes in stiffness, strength, and toughness by up to 1800-, 80-, and 20-folds, respectively, when the temperature is raised from 25 to 70 °C, while the volumes of the gels are almost unchanged. As a potential application, the performance of the materials as athletic protective gear is demonstrated. This work provides a pathway for developing thermally stiffened materials and may significantly broaden the scope of polymer applications.