Abstract: With changes in modern lifestyles, weakened immunity has become a health problem for an increasing number of people. The development of natural active products with immunomodulatory activity has become a popular research topic. Marine sulfated polysaccharides are natural active products that are rich in sulfate groups and are extracted and isolated from marine organisms. They have a unique structure and multiple biological activities and have shown great potential with regard to having immune regulatory activity. To better understand the effects of marine sulfated polysaccharides on immune regulatory activity and the associated underlying mechanisms, this article was designed to analyse and summarize the current research literature on marine sulfated polysaccharides. The types of marine sulfated polysaccharides are summarized based on their structure and source, with a focus on functions involving regulating macrophages, natural killer cells, T/B lymphocytes, complement systems, and gut microbiota. The immune regulatory activity of marine sulfated polysaccharides and the underlying mechanisms are summarized. This article provides a theoretical basis for subsequent research on the immunomodulatory activity and structure-activity relationships of marine sulfated polysaccharides and provides novel ideas for the development of new marine sulfated polysaccharide immunoenhancers. In the future, functional foods or special medical foods based on marine sulfated polysaccharides designed to treat immune deficiency, tumours, and autoimmune diseases may be developed, and these foods will play a positive role in promoting the development of the food industry and human health.