Marine biotechnology is an evolving field that employs marine organisms to generate, alter, or enhance biological resources for nutritional, industrial, medicinal, or environmental purposes. Given the pharmacological importance of marine bioactive compounds, numerous metabolic products derived from marine biomass were examined for their medicinal efficacy. The identification of algal polysaccharides exhibiting antiviral properties has notably risen over the last few decades. Additionally, distinct polysaccharides derived from seaweeds, including carrageenan, alginates, fucoidans, galactans, laminarins, and ulvans, have demonstrated efficacy in combating viral infection. New antiviral therapies could be developed using seaweed polysaccharides because of their various characteristics that are similar to effective antiviral drugs. In this chapter, we describe the most important marine seaweed polysaccharides with potential antiviral activities and the mechanism of their action as well as examples of their successful use in the treatment of various viral infections.