结晶水是以中性水分子形式参加到晶体结构中去的一定量的水；在晶格中占有一定的位置，水分子数量与矿物的其他成分之间常呈简单比例。

不同的含水化合物有特定的脱水温度，绝大部分伴有显着的吸热效应；土壤中土粒所含的结晶水，不能直接参加土壤中进行的物理作用，也不能被植物直接吸收。

晶体从溶液当中结晶时所析出时结合一定的数目的水分子，它原本是和其他化合物以分子的型态接和在一起的水，因此不具有水的特性，这类的晶体有水合氧化铁、氯化亚钴、硫酸铜晶体等。

In chemistry, water of crystallization or water of hydration or crystallization water is water that occurs inside crystals. Water is often necessary for the formation of crystals. In some contexts, water of crystallization is the total weight of water in a substance at a given temperature and is mostly present in a definite (stoichiometric) ratio. Classically, "water of crystallization" refers to water that is found in the crystalline framework of a metal complex or a salt, which is not directly bonded to the metal cation