Selenium polysaccharides are a new type of functional polysaccharide that combines inorganic selenium with polysaccharides to form an organic selenium product. Selenium polysaccharides are obtained using three different methods, have no toxicity or side effects, and are easily absorbed and utilized by the body. A number of studies have demonstrated that selenium polysaccharides possess better antioxidant, antitumour, immune regulation, hypoglycaemic, and heavy metal removal activities than that of either polysaccharides or inorganic selenium. Selenium polysaccharides have gradually become a research topic of interest for the development of functional foods and pharmaceutical products. However, further studies are required to investigate the structures and mechanisms of selenium polysaccharides. At present, reviews that focus on the bioactivities of selenium polysaccharides are lacking. The aim of this study was to summarize the selenium polysaccharide bioactivity reports from the past decades, describe the mechanisms and shortcomings of these studies, and evaluate the need for further development.