Brackish water (less commonly brack water) is salt water and fresh water mixed together. It is saltier than fresh water, but not as salty as seawater. It may result from mixing of seawater with fresh water, as in estuaries, or it may occur in brackish fossil aquifers.

Some human activities can produce brackish water, mostly certain civil engineering projects such as dikes and the flooding of coastal marshland to produce brackish pools for freshwater prawns.