In situ gellable hydrogels are an innovative and promising technique among gastroretentive drug administration with the objective to overcome the limitations associated with conventional oral dosage forms, for example, poor bioavailability and limited gastric residence times. These systems are delivered as liquids which exhibit a sol–gel transition when exposed to specific physiological conditions in the stomach, like pH, temperature, ionic changes, and solvent exchange. The produced gel, which is typically lighter than gastric fluids and/or bioadhesive, floats, or sticks to the gastrointestinal mucosa, and eventually extends gastric retention time. Such prolonged retention results in more controlled and sustained drug release, improving the bioavailability of drugs with limited absorption windows or those that are unstable under intestinal environments. These in situ gels are made from a variety of natural and synthetic polymers, such as chitosan, gellan gum (GG), sodium alginate (SA), pectin, and poly (lactic acid) derivatives. The chapter addresses the formulation techniques, gelation mechanisms, assessment methods, and therapeutic benefits of these systems, emphasizing their ability to enhance therapeutic outcomes and patient compliance. It also discusses the pharmacological importance of gastroretentive drug delivery systems (GRDDS), formulation difficulties, and future prospects for the clinical application of in situ gellable hydrogels as successful gastroretentive technologies.