Red yeast rice or red rice koji is a bright reddish purple fermented rice, which acquires its color from being cultivated with the mold Monascus purpureus. Red yeast rice is what is referred to as a kōji in Japanese, meaning "grain or bean overgrown with a mold culture", a food preparation tradition going back to ca. 300 BC.

In addition to its culinary use, red yeast rice is also used in Chinese herbology and Traditional Chinese medicine, possibly during the Tang dynasty around AD 800. Red yeast rice is described in the Chinese pharmacopoeia Ben Cao Gang Mu by Li Shizhen.

A modern-era use as a dietary supplement developed in the late 1970s after researchers were isolating lovastatin from Aspergillus and monacolins from Monascus, the latter being the same fungus used to make red yeast rice. Chemical analysis soon showed that lovastatin and monacolin K were identical. Lovastatin became the patented prescription drug Mevacor. Red yeast rice went on to become a non-prescription dietary supplement in the United States and other countries. In 1998, the U.S. Food and Drug Administration (FDA) initiated action to ban a dietary supplement containing red yeast rice extract, stating that red yeast rice products containing monacolin K are identical to a prescription drug, and thus subject to regulation as a drug.

Red yeast rice is also known as red fermented rice, red kojic rice or red koji rice from its Japanese name, and anka or angkak from Southern Min pronunciations of its Chinese name. In both the scientific and popular literature in English that draws principally on Japanese traditional use, red yeast rice is most often referred to as "red rice koji". English language articles favoring Chinese literature sources prefer the translation "red yeast rice".

Red yeast rice is produced by cultivating the mold species Monascus purpureus on rice for 3–6 days at room temperature. The rice grains turn bright red at the core and reddish purple on the outside. The fully cultured rice is then either sold as the dried grain, or cooked and pasteurized to be sold as a wet paste, or dried and pulverized to be sold as a fine powder. China is the world's largest producer of red yeast rice, but European companies have entered the market.

Red yeast rice is used to color a wide variety of food products, including fermented tofu, red rice vinegar, char siu, Peking duck, and Chinese pastries that require red food coloring.

In China, documentation dates back to at least the first century AD. It is also traditionally used in the production of several types of Chinese huangjiu (Shaoxing jiu), and Japanese sake (akaisake), imparting a reddish color to these wines. It was called a "koji" in Japanese, meaning "grain or bean overgrown with a mold culture".

The lees left over from wine production, known as hóngzāo (红糟), can be used as flavoring, imparting a subtle but pleasant taste to food. The lees are particularly commonly used in Fujian cuisine, where they are used for dishes like Fujian red wine chicken, a celebratory dish associated with birthdays and Chinese New Year.