Aspergillus oryzae, also known as kōji mold (Japanese: ニホンコウジカビ (日本麹黴), Hepburn: nihon kōji kabi), is a filamentous fungus (a mold) used in East Asia to saccharify rice, sweet potato, and barley in the making of alcoholic beverages such as sake and shōchū, and also to ferment soybeans for making soy sauce and miso. However, in the production of fermented foods of soybeans such as soy sauce and miso, Aspergillus sojae is sometimes used instead of A. oryzae. Incidentally, in China and Korea, the fungi used for fermented foods for a long time in the production of traditional alcoholic beverages were not A. oryzae but fungi belonging to Rhizopus and Mucor. A. oryzae is also used for the production of rice vinegars. Barley kōji (麦麹) or rice kōji (米麹) are made by fermenting the grains with A. oryzae hyphae.

Genomic analysis has led some scholars to believe that the Japanese domesticated the Aspergillus flavus that had mutated and ceased to produce toxic aflatoxins, giving rise to A. oryzae. Eiji Ichishima of Tohoku University called the kōji fungus a "national fungus" (kokkin) in the journal of the Brewing Society of Japan, because of its importance not only for making the kōji for sake brewing, but also for making the kōji for miso, soy sauce, and a range of other traditional Japanese foods. His proposal was approved at the society's annual meeting in 2006.

The Japanese word kōji (麹) is used in several meanings, and in some cases it specifically refers to A. oryzae and A. sojae, while in other cases it refers to all molds used in fermented foods, including Monascus purpureus and other molds, so care should be taken to avoid confusion.