Kvass is a fermented beverage made from black or regular rye bread. The colour of the bread used contributes to the colour of the resulting drink. It is classified as a non-alcoholic drink by Russian standards, as the alcohol content from fermentation is typically less than 1.2%. Overall, the alcohol content is low (0.05% - 1.0%). It is often flavoured with fruits or herbs such as strawberries, raisins or mint. Kvass is also used for preparing a cold summertime soup called okroshka.

It is popular in Russia, Belarus, Ukraine, Lithuania, Latvia, Estonia, and other Eastern and Central European countries as well as in former Soviet states, such as Kazakhstan and Uzbekistan, where one can see many kvass vendors in the streets. Kvass is also popular in Harbin and Xinjiang, areas within China that are influenced by Russian culture.

The word "Kvass" derived from Old East Slavic квасъ, kvasŭ, meaning "yeast" or "leaven". Today the words used are almost the same: in Belarusian: квас, kvas; Chinese: 格瓦斯/克瓦斯, géwǎsī/kèwǎsī; Latvian: kvass; Polish kwas chlebowy; Russian: квас, kvas; in Ukrainian: квас/хлібний квас/сирівець, kvas/khlibnyy kvas/syrivets. Except Lithuanian: gira, which means beverage similar to Latvian dzira.