海藻酸又称藻酸、褐藻酸、海藻素，是存在于褐藻细胞壁中的一种天然多糖。通常纯品为白色到棕黄色纤维、颗粒或粉末。海藻酸易与阳离子形成凝胶，如海藻酸钠等，被称为海藻胶、褐藻胶或藻胶。

海藻酸(Alginic acid)是由单糖醛酸线性聚合而成的多糖，单体为β-D-甘露醛酸（M）和 α-L-古罗糖醛酸（G）。M和G单元以M-M，G-G或M-G的组合方式通过1，4糖苷键相连成为嵌段共聚物。海藻酸的实验式为(C₆H₈O₆)_n，分子量范围从1万到60万不等。

Alginic acid, also called algin or alginate, is an anionic polysaccharide distributed widely in the cell walls of brown algae, where through binding with water it forms a viscous gum. Its colour ranges from white to yellowish-brown. It is sold in filamentous, granular or powdered forms.