The interaction of water with Fe3O4(001) is studied by density functional theory calculations including an on-site Coulomb term. For isolated molecules, dissociative adsorption is strongly promoted at surface defect sites, while at higher coverages a hydrogen-bonded network forms with alternating molecular and dissociated species. This mixed adsorption mode and a suppression of the (2×2)R45°reconstruction are confirmed by a quantitative low energy electron diffraction analysis. Adsorbate induced electron transfer processes add a new dimension towards understanding the catalytic activity of magnetite(001).