Marine polysaccharides have emerged as versatile and biocompatible materials for the development of injectable drug delivery systems, offering a sustainable alternative to synthetic polymers. Derived from seaweed and marine organisms, polysaccharides such as alginate, chitosan, carrageenan, and fucoidan possess unique physicochemical properties, including biodegradability, gel-forming ability, and tunable viscosity, making them ideal candidates for parenteral applications. Their structural diversity allows for controlled drug release, targeted delivery, and enhanced therapeutic efficacy while minimizing systemic side effects. Recent advances have demonstrated their potential in encapsulating a wide range of therapeutics, including proteins, peptides, and small molecules, with promising results in cancer therapy, tissue engineering, and immunomodulation. Furthermore, marine polysaccharides are amenable to chemical modification and nanoparticle formulation, broadening their application in precision medicine. This review highlights the current progress, challenges, and future directions in utilizing marine-based polysaccharides for injectable drug delivery systems, underscoring their role in the advancement of biocompatible and effective pharmaceutical platforms.