Kvass ("leaven") is a Russian fermented non-alcoholic or mildly alcoholic beverage.

It is made by the fermentation of whatever grain is available, such as wheat, rye or barley (Common or Table Kvass), or of black or rye bread, with the addition of sugar or fruit, usually appless. It has been a common drink in Russia since the 16th century. Though in the cities it is made commercially, usually it is home made.

In strength Kvass can be almost non-alcoholic and at its strongest is only around 2.2%. It can often be flavoured with other fruits or herbs such as strawberries or mint. The main benefit of kvass is that for much of Russian history it has been safer to drink kvass than water.