Cardiovascular diseases continue to impose the highest mortality burden globally, with myocardial infarction being a principal contributor due to the irreversible loss of cardiomyocytes and subsequent ventricular remodeling. Despite advancements in pharmaceutical and surgical interventions, current treatments remain inadequate for restoring cardiac function. In this context, in situ gellable hydrogels have emerged as a transformative strategy in cardiac tissue engineering, offering biomimetic platforms that provide mechanical reinforcement, deliver bioactive agents, and support tissue regeneration. This chapter presents a comprehensive overview of in situ gellable hydrogels, with an emphasis on their primary design considerations, gelation mechanisms, and multifaceted roles in cardiac tissue engineering. To ensure their effectiveness in clinical applications, several critical factors must be addressed during the design and development process, including biocompatibility, biodegradability, mechanical integrity, stimuli responsiveness, and injectability with optimal flow characteristics. Recent innovations incorporating conductive, antioxidant, and self-healing components are highlighted for their roles in enhancing electrical coupling, reducing oxidative stress, and improving cell retention in postinfarct myocardium. These attributes facilitate a wide range of therapeutic applications, including myocardial regeneration, prevention of adverse remodeling, restoration of electrophysiological function, and sustained delivery of cells and therapeutic agents. The synergy between biomaterials and bioactive payloads such as stem cells, extracellular vesicles, growth factors, and gene delivery systems is also discussed, underscoring their potential to modulate the hostile postmyocardial infarction microenvironment. Despite encouraging preclinical and clinical outcomes, challenges remain in translating these materials into clinical settings. These challenges include optimizing gelation kinetics, promoting vascularization, and addressing regulatory considerations. Future studies are anticipated to focus on developing smart hydrogel systems that exhibit dynamic responsiveness and prolonged integration within cardiac tissue. These advancements have the potential to transform clinical strategies, effectively bridging the gap between biological complexity and engineering capabilities in regenerative medicine.