The practical applications of Li‐ion batteries (LIBs) are challenged by their safety concerns when using liquid electrolytes (LEs). Solid‐state gel polymer electrolytes (GPEs) can address this challenge and have drawn increased attention recently. Normally, GPEs are prepared separately and then assembled into cells, which undoubtedly result in dissatisfactory solid/solid interfacial compatibility and low ionic conductivity. Fortunately, in situ GPEs are proposed to address the above challenges and simplify the preparation process. Typically, LE precursor is injected into the cells and gradually transformed into a quasi‐solid gel state under the conditions of thermal or chemical initiators. Consequently, the obtained in situ GPEs could fully infiltrate the electrode and better interface contact of gel electrolyte/electrode is thus inherited. In this review, the authors focus on the in situ GPEs used in lithium batteries (LBs), and summarize recent progress of the design, synthesis, and applications of in situ GPEs. Based on the different ways of triggering polymerization, there are mainly three methods: thermochemical gelation, polymerization by additional chemical initiators, and cross‐linking initiated by LiO bond. Composite GPEs based on in situ solidification method are introduced as a promising strategy to improve the electrochemical performances. Finally, up‐to‐date research progresses are discussed, and perspectives are provided on the development and challenges of in situ GPEs to meet the requirements for their practical applications in LBs. In situ gel polymer electrolytes (GPEs) are being widely concerned as high‐priority materials for safe and high‐performance lithium batteries (LBs). This review intends to introduce in situ GPEs by reviewing their designs and applications in LBs. The challenges and prospects of in situ GPEs are proposed from the perspective of promoting the practical applications of this promising field.