Potassium peroxocarbonate hemihepta peroxohydrate, K2(O2)CO2·3.5H2O2, crystallizes in the monoclinic space group C2/c (No. 15) with Z = 8 and the lattice parameters a = 17.362(3) Å, b = 8.451(1) Å, c = 13.195(2) Å, β = 112.71(1)° und V = 1786.0(5) Å3. The salt of the monoperoxocarbonic acid could be obtained as colorless needles in a concentrated solution of hydrogen peroxide and potassium hydrogen carbonate. The structure consists of two crystallographic different potassium atoms, one peroxocarbonate group and five hydrogen peroxide molecules. The K–O bond lengths are in the range of 2.699–3.291 Å. Along the c‐axis [(K+)(H2O2)2] layers alternate with [(H2O2)(O2CO2)2–] layers. Along [010] the peroxocarbonate group acts as a mono‐ and bidentate ligand towards one potassium atom. The structure is stabilized via hydrogen bonds with donor‐acceptor‐distances from 2.567 to 2.796 Å and hydrogen bond angles from 163.3 to 178.9°. A Raman spectrum has been measured to confirm the structure.