The synthesis of pure birnessite (δ-MnO2) by a simple, fast and reliable method is introduced. Birnessite was synthesized by a sol–gel reaction involving the reduction of KMnO4 by lactate at ambient pressure and temperature. Our method does not include the use of strong acids and bases and does not produce toxic residues. The results from both X-ray diffraction and Fourier-transform infrared spectroscopy, compared with a synthesized reference material, verified our synthesized mineral as birnessite. Secondary electron images showed a micro-porous structure of aggregated birnessite colloids, and consistently, we measured a very large specific BET surface area of 249.15±0.05 m2 g−1. Birnessite is known to have a variable mineral formula, and the elemental composition of our mineral is within the range of other birnessites. Birnessite obtained by this procedure could serve as a model mineral for laboratory experiments on the biogeochemistry of Mn, and furthermore it may also have a potential as electron acceptor for remediation purposes.