Three-dimensional networks of polymers called hydrogels have the ability to absorb vast volumes of water or biological fluid. Since their introduction as soft contact lenses in the 1960s, their use in the medical field has significantly expanded. Many hydrogels are widely used in both laboratory studies and clinical applications. Smart hydrogels, also known as environmentally responsive hydrogels, are materials capable of reversible changes in response to single or multiple external stimuli. Smart hydrogels are the theme of significant and competitive research in various scientific fields, including biomaterials, tissue engineering, biotechnology, and pharmaceuticals. Due to their chemical design diversity, they are utilized as next-generation materials in smart and multi-drug carriers, soft biomimetic structures, biorecognition and bioseparation systems, biosensors, nanoscale devices, nanorobots, tissue engineering scaffolds, injectable systems, and as 3D and 4D printing bioinks.