Kombucha (also tea mushroom, tea fungus, or Manchurian mushroom when referring to the culture; Latin name Medusomyces gisevii) is a fermented, effervescent and sweetened black tea drink. Sometimes the beverage is called kombucha tea to distinguish it from the culture of bacteria and yeast. Juice, spices, fruit, or other flavorings are often added. Commercial kombucha contains small amounts of alcohol.

Kombucha is believed to have originated in China, where the drink is traditional. While it is named after the Japanese term for kelp tea in English, the two drinks have no relation. By the early 20th century kombucha spread to Russia, then other parts of Eastern Europe and Germany. Kombucha is now homebrewed globally, and also bottled and sold commercially. The global kombucha market was worth approximately US$1.7 billion as of 2019^[update].

Kombucha is produced by symbiotic fermentation of sugared tea using a symbiotic culture of bacteria and yeast (SCOBY) commonly called a "mother" or "mushroom". The microbial populations in a SCOBY vary. The yeast component generally includes Saccharomyces cerevisiae, along with other species; the bacterial component almost always includes Gluconacetobacter xylinus to oxidize yeast-produced alcohols to acetic acid (and other acids). Although the SCOBY is commonly called "tea fungus" or "mushroom", it is actually "a symbiotic growth of acetic acid bacteria and osmophilic yeast species in a zoogleal mat [biofilm]". The living bacteria are said to be probiotic, one of the reasons for the popularity of the drink.

Numerous health benefits have been claimed to correlate with drinking kombucha; there is little evidence to support any of these claims. The beverage has caused rare serious adverse effects, possibly arising from contamination during home preparation. It is not recommended for therapeutic purposes.

Kombucha likely originated in the Bohai Sea region of China. In 20th century China, some traditional Chinese medicine practitioners used it as a remedy for lung and stomach ailments, but the drink was not widely known across the country. It spread to Russia before reaching Europe and gained popularity in the United States in the early 21st century. In the intervening years, its popularity in the West eclipsed its popularity in China, where it remains less known, though consumption is increasing in many East Asian countries. With an alcohol content under 0.5%, it is not federally regulated in the U.S.

There are numerous myths surrounding the history of kombucha, likely invented in the West as a marketing gimmick. There is no evidence that kombucha originated in Qin dynasty China and that it was served to Emperor Qin Shi Huang. Furthermore, there is no evidence that kombucha was ever present in Korea or Japan before the late 20th century, making claims that a Korean doctor named Kombu served the drink to Japanese Emperor Ingyo entirely apocryphal.

Kombucha was first introduced to Japan from Russia and became a health fad in the country following the publication of Sumako Nakamitsu's 1974 bestseller, Kōcha Kinoko Health Methods. Although kombucha had already been consumed in China before it became known in Japan, overseas interest in the beverage for its purported wellness benefits contributed to increased attention and consumption in China during the 1980s.

Prior to 2015, some commercially available kombucha brands were found to contain alcohol content exceeding this threshold, sparking the development of new testing methods. With rising popularity in developed countries in the early 21st century, kombucha sales increased after it was marketed as an alternative to beer and other alcoholic drinks in restaurants and pubs.