A description of the new mineral innsbruckite, Mn33(Si2O5)14(OH)38, a hydrous manganese phyllosilicate found in Tyrol, Austria is given. The crystal structure was determined by single-crystal synchrotron radiation diffraction experiments at the X06DA beamline at the Swiss Light Source (Paul Scherrer Institute, Villigen, Switzerland). The space group is Cm and lattice parameters are a = 17.2760(19), b = 35.957(5), c = 7.2560(8) Å, β = 91.359(7)°, V = 4506.1(10) Å3, Z = 2. Innsbruckite belongs to the group of modulated 1:1 layer silicates and is chemically and structurally quite closely related to bementite, Mn7(Si2O5)3(OH)8. The chemical analysis revealed a close to ideal composition with only minor amounts of Al, Fe and Mg. Using Liebau's nomenclature for silicate classification the silicate anion can be described as an unbranched siebener single layer. Innsbruckite shows a complex topology of the silicate sheet, exhibiting 4-, 5-, 6- and 8-membered rings. The silicate sheet is fully characterized using vertex symbols, and its topology is compared to those in other complex sheet silicates. Furthermore, the structural investigation is complemented with Raman spectroscopic studies.