Saccharina japonica or kombu (/ˈkɒmbuː/, from Japanese konbu referring to the algae in the family Laminariaceae) is a marine species of Phaeophyceae (brown algae), a type of kelp or seaweed, that is extensively cultivated on ropes in between the seas of Japan and Korea. It is widely eaten in East Asia. A commercially important species, Saccharina japonica is known as called ma-konbu (真昆布) in Japanese, dasima (다시마) in Korean, and hǎidài (海带) in Chinese. Large harvests are produced by rope cultivation, a simple method of growing seaweeds by attaching them to floating ropes in the ocean.

The species has been cultivated in China, Japan, Russia, France, and Korea. It is one of the two most consumed species of kelp in China and Japan. The harvest is also used for the production of alginates, with China producing up to ten thousand tons of the product each year.

Consuming excessive amounts of Saccharina japonica has been shown to suppress thyroid function.

The species was transferred to Saccharina in 2006. Three synonyms for this species name are Laminaria japonica J.E. Areschoug 1851, its variety Laminaria japonica var. ochotensis (Miyabe) Okamura 1936, and Laminaria ochotensis Miyabe 1902.

With the development of cultivation technology, over 90% of Japanese kombu is cultivated, mostly in Hokkaidō, but also as far as south of the Seto Inland Sea.