克瓦斯（古教会斯拉夫语：квасъ，转写：kvasŭ，来源于原始斯拉夫语*kvasъ，意思为“酵母”或“发酵”，现代俄语、乌克兰语及白俄语：квас，波兰语：kwas chlebowy，拉脱维亚语：kvass，立陶宛语：gira，意思为“饮料”，爱沙尼亚语：kali，意思为“发酵”）是一种盛行于俄罗斯、乌克兰等东欧国家的一种低度酒精的饮料，口感酸甜，有少量二氧化碳，以黑麦面包发酵而成。由于酒精含量低，通常最多1.2%甚至无酒精，因此儿童也可以饮用，广受欢迎。饮料常以水果或香草调味（例如草莓或薄荷）。俄国人在夏天也以它制作俄罗斯冻汤（окрошка）。

Kvass is a traditional Slavic and Baltic fermented beverage commonly 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 standard, as the alcohol content from fermentation is typically less than 1.2%. Generally, the alcohol content is low (0.5–1.0%). It may be flavoured with fruits such as strawberries and raisins, or with herbs such as mint.