Switzerite, Mn3(PO4)2·7H2O, is monoclinic, P21/a, with a = 8.528(4) Å, b = 13.166(5) Å, c = 11.812(4) Å, β = 110.05(3)°, and Z = 4. A crystal sealed in a glass capillary tube was used to gather 1634 unique observed reflections, using a Philips PW1100 four-circle diffractometer. The structure was solved by direct methods with a final R of 0.089. The major structural unit in switzerite is a sheet of composition Mn4O10(H2O)4, which lies parallel to (001), and which is similar to the atomic sheet found in metaswitzerite, Mn3(PO4)2·4H2O. MnO3(H2O)3 octahedra are linked to this sheet by vertex-sharing, forming a complex slab of composition [Mn6O14(H2O)10]2. Phosphate groups knit together the slabs internally to form a neutral layer with composition Mn6(H2O)10(PO4)4, two per unit cell. Between these neutral layers are free water molecules, eight per unit cell, bonded to the layers by H bonds. Switzerite tranforms to metaswitzerite by expulsion of ¾ of the free water molecules and condensation of the neutral layers along c into a framework structure. Only minor changes occur within the layers, and the transformation of switzerite into metaswitzerite can be considered a typical topotactic reaction.