米曲菌（学名：Aspergillus oryzae），又名米曲霉菌、米曲霉、曲霉菌或曲霉菌。日文名「麹菌」。米曲菌是一种带有菌丝（hypha）的真菌与霉菌，在中国与日本料理中经常被用来发酵大豆来制作酱油、味噌与甜面酱。而这种真菌在上述两个文化中也被用于糖化（saccharify）稻米、马铃薯、麦等粮食来发酵制作酒类，像是黄酒、清酒、泡盛与烧酎等等。除此之外，米曲菌这种真菌也被用在米醋（rice vinegars）的制作上，如日本的「合わせ酢」（awasezu）等。而由此种真菌产生出来的蛋白酶被诺维信（Novozymes）以「风味蛋白酶」（Flavourzyme）的名称于市场上销售。日本也将米曲菌认定为日本的「国菌」。

Aspergillus oryzae, known in English as kōji (from Japanese kōji 麹), is a filamentous fungus (a mold) used in Chinese and other East Asian cuisines to ferment soybeans for making soy sauce and fermented bean paste, and also to saccharify rice, other grains, and potatoes in the making of alcoholic beverages such as huangjiu, sake, makgeolli, and shōchū. The domestication of A. oryzae occurred at least 2000 years ago. A. oryzae is also used for the production of rice vinegars.