Agar (a-gar)' or agar agar ' is a gelatinous substance chiefly used as a solid substrate to contain culture medium for microbiological work. It is an unbranched polysaccharide obtained from the cell membranes of some species of red algae or seaweed. Commercially it is derived primarily from Gelidium amansii.

Agar can be used as a laxative, a vegetarian gelatin substitute, a thickener for soups, in jellies, ice cream and Japanese desserts such as anmitsu, as a clarifying agent in brewing, and for paper sizing fabrics. The word agar comes from the Malay word agar-agar (meaning jelly). It is also known as kanten, agal-agal (Ceylon agar), or China grass. Chemically, agar is a polymer made up of subunits of the sugar galactose. Agar polysaccharides serve as the primary structural support for the algae's cell walls.

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