Hydrogels are three-dimensional polymer networks that contain a large amount of water inside. Certain hydrogels can be injected in solution and transformed into the gel state with the required shape. Despite their potential biomedical applications, the use of hydrogels has been severely limited because all the conventional hydrogels inevitably "swell" under physiological conditions, which drastically degrades their mechanical properties. We report the synthesis of injectable "nonswellable" hydrogels from hydrophilic and thermoresponsive polymers, in which two independently occurring effects (swelling and shrinking) oppose each other. The hydrogels can endure a compressive stress up to 60 megapascals and can be stretched more than sevenfold without hysteresis. Our results demonstrate that the suppression of swelling helps retain the mechanical properties of hydrogels under physiological conditions.