Hydrogel-integrated optical fiber sensors have garnered momentous interest due to their optical properties, biocompatibility, and biodegradability. Integrating active materials with hydrogels facilitates hydrogels to be employed as a smart material that responds to external stimuli in biological or chemical media, allowing them to be a potential candidate for sensing applications. Although substantial research efforts have been made on integrating hydrogels with optical fibers, this field has never been reviewed extensively. In this paper, hydrogel-integrated optical fiber sensors are reviewed comprehensively, including the fundamentals of hydrogels, the synthesis process, and the working principle of various sensing methods. The light-guiding and sensing mechanism adopted by hydrogel-integrated optical fibers is discussed. The current progress in their fabrication procedures is categorized and explained. Finally, we have briefly evaluated the applications of hydrogel-integrated optical fiber sensors in various fields spanning from biosensing to environmental sensing.