Birch sap or birch water is the sap directly tapped from birch trees, Betula alba (white birch), Betula pendula (silver birch), Betula lenta, Betula papyrifera, and Betula fontinalis.

Birch sap is collected only at the break of winter and spring when the sap moves intensively. When fresh, it is a clear and uncoloured liquid, very similar to water, often slightly sweet with a slightly silky texture. After two to three days, the sap starts fermenting and the taste becomes more acidic. Birch sap may be consumed both fresh and naturally fermented.