In crystallography, water of crystallization or water of hydration or crystallization water is water that occurs in crystals. Water of crystallization is essential for the maintenance of crystalline properties, but capable of being removed by sufficient heat. It is the total weight of water retained by certain salts at a given temperature and is mostly there in a definite (Stoichiometry) ratio. Classically, "water of crystallization" refers to water that is found in the crystalline framework of a metal complex but which is not directly bonded to the metal ion.

Compared to inorganic salts, proteins crystallize with unusually large amounts of water in the crystal lattice. A water content of 50% is not uncommon. The extended hydration shell is what allows the protein crystallographer to argue that the conformation in the crystal is not too far from the native conformation in solution.