Agar or agar-agar, is a jelly-like substance, obtained from red algae.

Agar is a mixture of two components: the linear polysaccharide agarose, and a heterogeneous mixture of smaller molecules called agaropectin. It forms the supporting structure in the cell walls of certain species of algae, and is released on boiling. These algae are known as agarophytes, and belong to the Rhodophyta (red algae) phylum.

Agar-agar is approximately 80% dietary fiber, so it can serve as an intestinal regulator. Its bulking quality has been behind fad diets in Asia, for example the kanten (the Japanese word for agar-agar) diet. Once ingested, kanten triples in size and absorbs water. This results in the consumers feeling fuller. This diet has recently received some press coverage in the United States as well. The diet has shown promise in obesity studies.