The role of metastable nanocrystalline precursors, like schwertmannite, in iron and sulfate rich acidic waters is commonly underestimated or even neglected. In addition to schwertmannite metastability, its heterogeneous chemical composition and the current use of disparate solubility products result in an incongruous understanding of this mineral. In order to characterize schwertmannite stability in acid mine drainage settings, we used coincident schwertmannite and solution samples to determine how its solubility product is related to its composition. The solubility products (as log Ksp) for 30 natural samples of this study span a range of log Ksp values from 5.8 to 39.5. These values show a gradual distribution on the pH–pe space from pH 1.93 to 4.71 and pe values from 8.5 to 13.7. A set of three predictive equations to select the best schwertmannite solubility product for each new specific case study was obtained. This approximation allows generating an appropriate solubility product for schwertmannite despite the lack of information in certain environments (e.g., absence of former water chemistries on Mars). The trend observed for Fe and S contents in schwertmannite can be interpreted as a pseudo-solid solution ranging from high to low S and Fe concentrations. The polyphasic nature of schwertmannite was studied by means of a thermodynamic model assuming equilibrium between a hydrous ferric oxide (HFO), schwertmannite, and solution. All the results obtained in this study support the understanding of schwertmannite as a polyphasic nanomineral and encourage using a broad log Ksp range to model the solubility of schwertmannite in nature.