In chemistry, a topotactic transition involves a structural change to a crystalline solid, which may include loss or gain of material, so that the final lattice is related to that of the original material by one or more crystallographically equivalent, orientational relationships.

An example is a transition in which the relative structure of the anionic array is unaltered but the cations reorganize as in:

β-Li2ZnSiO4 ⇒ γ-Li2ZnSiO4

An alternate example is the oxidation of magnetite to maghemite.