Marine polysaccharides have promising prospects and a very important potential that could contribute to the development of a marine bioindustry. Specifically, polysaccharides obtained from microalgae and marine seaweeds have attracted a great deal of attention in the last few years, in view of their diversity and biological activities. For this reason, these biomolecules have a wide variety of industrial applications, including health, nutraceuticals, environment, and agriculture. The beneficial biological activities are related to the chemical composition and structure of these molecules such as molecular weight, monosaccharide composition, uronic acids, and sulfate content. This chapter focuses on the extraction and purification technologies of marine polysaccharides. The diversity of these polysaccharides and their biological activities are discussed with a focus on their applications in new fields such us plant biostimulants. Finally, structure–function relationships of marine polysaccharides underlines the importance of multidisciplinary approaches in order to improve industrial applications of these biomolecules.