The crystal structure of the mineral koninckite was solved from synchrotron powder X-ray diffraction (XRD) data and refined using density-functional theory (DFT) calculations. Koninckite is tetragonal, with the space group P41212, a = 11.9800(5) Å, c = 14.618(1) Å, V = 2097.9(2) Å3, Z = 8. Its structure is a heteropolyhedral framework with zeolite-like tunnels along . Owing to the severe peak overlap in the powder XRD data and the probable intergrowth of enantiomorphic domains in koninckite, the DFT calculations were applied to provide precise atomic positions (including hydrogen). Additionally, the DFT calculations suggest strongly that koninckite is an antiferromagnetic semiconductor, at least at low temperatures. The DFT computations were used to locate H2O molecules in the channels and to complete the structural description. Thermogravimetric analysis and powder XRD data at variable temperatures show that the structure of koninckite dehydrates and eventually collapses between 160–180°C. Negative thermal expansion was observed between 80 and 150°C. A list of the known occurrences of koninckite suggests that this mineral is not as rare as assumed previously; koninckite is often fine-grained, inconspicuous, and thereby easy to overlook. Koninckite is yet another natural example of an Fe-phosphate zeolitic material.