Koji is the Japanese name for the mold fungus Aspergillus oryzae. The spores of this fungus are called "Koji-Kin" (麹菌). Koji – similar to yeast – is a starter culture used to produce a variety of ferments. The fungus is essential for the production of miso, soy sauce, amazake, sake, mirin, rice vinegar, and other traditional Japanese seasonings. Because Aspergillus oryzae forms the absolute foundation of Washoku culture (traditional Japanese cuisine), it was designated Japan's official national microorganism by the "Brewing Society of Japan" in 2006.

The use of koji in Japan has a long history: the first written mention outside China dates back to 725 AD in the Japanese document Harima no Kuni Fudoki. However, the practice likely originated in China, where the precursor of koji was documented as early as the 4th century BC.

But why ferment with koji? As it grows on rice or other substrates, koji mold produces a variety of enzymes (such as amylases and proteases). These enzymes digest the substrate by breaking down complex starches into sugars and proteins into amino acids. This creates an infinite variety of flavors during the fermentation process and gives miso and soy sauce their rich, delicious taste (umami).