Tumor immunotherapy, a revolutionary cancer treatment, is hindered by inadequate immune cell activation, immunosuppressive tumor microenvironment (TME), and off-target toxicities of immunotherapeutics. These bottlenecks necessitate innovative strategies to enhance efficacy and reduce side effects. Marine polysaccharides have garnered significant attention due to their potential to enhance immune cell activity and regulate the tumor microenvironment, among other benefits. Due to their excellent biocompatibility, modifiability, and relatively low cost, polysaccharides are increasingly being explored as materials for drug delivery systems. The development of marine polysaccharide-based drug delivery systems represents an opportunity for advancing tumor immunotherapy. This review focuses on the application of marine polysaccharide drug delivery systems in tumor immunotherapy, exploring the mechanisms underlying the bioactivity of marine polysaccharides, the design of drug delivery systems, and the interactions between these systems and tumor immunotherapy, aiming to provide a framework for advancing marine polysaccharide-based therapeutics, accelerating the clinical translation of effective, safe, and targeted tumor immunotherapy strategies.