<p>Injectable stimuli-responsive hydrogels have emerged as a highly versatile class of biomaterials designed to enhance precision in drug delivery, tissue engineering, and regenerative medicine. These hydrogels are engineered to undergo controlled sol–gel transitions or degradation in response to specific environmental stimuli, including temperature, pH, enzymes, reactive oxygen species (ROS), glucose, light, ultrasound, magnetic fields, and electricity. By leveraging these stimuli-responsiveness properties, hydrogels offer significant advantages, such as improved biocompatibility, minimally invasive administration, and the ability to provide localized and sustained therapeutic effects. In this review, a comprehensive analysis of recent developments in injectable stimuli-responsive hydrogels is provided, with a focus on their classification, fabrication techniques, and biomedical applications. Particular attention is given to their role in cardiovascular therapy, cancer treatment, and other emerging fields where targeted therapeutic interventions are required. Furthermore, existing challenges related to hydrogel stability, responsiveness, and clinical translation are discussed, along with potential strategies for overcoming these limitations. By summarizing key advancements and addressing future perspectives, this review aims to provide a valuable resource for researchers working toward the next generation of smart hydrogel-based biomedical solutions.</p>