Kvass is a fermented, cereal-based, low-alcoholic beverage of cloudy appearance and a sweet and sour taste.

Kvass originates from northeastern Europe, where grain production was considered insufficient for beer to become a daily drink. The first written mention of kvass is found in Primary Chronicle, describing the celebration of Vladimir the Great's baptism in 988.

Traditionally, kvass is made from a mash of rye bread or rye flour and malt soaked in hot water, fermented for about 12 hours with the help of sugar and bread yeast or baker's yeast at room temperature. In industrial methods, kvass is produced from wort concentrate combined with various grain mixtures. It is a drink known in Belarus, Estonia, Latvia, Lithuania, Moldova, Armenia, Georgia, Poland, Russia, and Ukraine. Kvass (or beverages similar to it) are also popular in some parts of China, Finland, Kazakhstan, and Uzbekistan.

The word kvass is ultimately from Proto-Indo-European base *kwh₂et- ('to become sour'). In English it was first mentioned in a text around 1553 as quass. Nowadays, the name of the drink is almost the same in most languages: in Polish: kwas chlebowy (lit. 'bread kvass', to differentiate it from kwas, 'acid', originally from kwaśny, 'sour'); Belarusian: квас, kvas; Russian: квас, kvas; Ukrainian: квас/хлібний квас/сирівець, kvas/khlibny kvas/syrivets; Latvian: kvass; Romanian: cvas; Hungarian: kvasz; Serbian: квас/kvas; Mandarin Chinese: 格瓦斯/克瓦斯, géwǎsī/kèwǎsī; Eastern Finnish: vaasa. Non-cognates include Estonian kali, Finnish kalja, Georgian ბურახი, burakhi; Latvian dzersis (lit. 'beverage'), Latgalian dzyra (lit. 'beverage', similar to Lithuanian gira), Lithuanian gira (lit. 'beverage', similar to Latvian dzira), and Swedish bröddricka (lit. 'bread drink').

In the traditional method, either dried rye bread or a combination of rye flour and rye malt is used. The dried rye bread is extracted with hot water and incubated for 12 hours at room temperature, after which bread yeast and sugar are added to the extract and fermented for 12 hours at 20 °C (293 K; 68 °F). Alternatively, rye flour is boiled, mixed with rye malt, sugar, and baker's yeast and then fermented for 12 hours at 20 °C (293 K; 68 °F).

The simplest industrial method produces kvass from a wort concentrate. The concentrate is warmed up and mixed with a water and sugar solution to create wort with a sugar concentration of 5–7% and pasteurized to stabilize it. After that, the wort is pumped into a fermentation tank, where baker's yeast and lactic acid bacteria culture is added, and the solution is fermented for 12–24 hours at 12 to 30 °C (285 to 303 K; 54 to 86 °F). Only around 1% of the extract is fermented out into ethanol, carbon dioxide, and lactic acid. Afterwards, the kvass is cooled to 6 °C (279 K; 43 °F), clarified through either filtration or centrifugation, and adjusted for sugar content, if necessary.

Initially, it was filled in large containers from which the kvass was sold on streets, but now, the vast majority of industrially produced kvass is filled and sold in 1–3-litre plastic bottles and has a shelf life of 4–6 weeks.

Kvass is usually 0.5–1.0% alcohol by weight, but may sometimes be as high as 2.0%.