The cell walls of green, red and brown seaweeds are dominated by the presence of sulfated polysaccharides. The recent genomic analyses of Chondrus crispus (Rhodophyta) and Ectocarpus siliculosus (Phaeophyceae) have revealed carbohydrate-active enzymes likely implicated in the biosynthesis of these polymers. The closest homologues of many of these algal enzymes are found in animals and are involved in the biosynthesis of sulfated glycosaminoglycans, indicating a shared ancestral pathway between animals, red and brown algae. Here, we review the structures of these algal sulfated polysaccharides and the current state of knowledge on the enzymes involved in their biosynthesis and remodeling. Based on the experimental and genomic data available, biosynthetic pathways for these essential cell wall polysaccharides are presented.