The low-temperature (LT) dependent behavior of a synthetic alunogen sample with composition Al2(SO4)3·16.61H2O has been studied in the overall temperature range from −100 to 23 °C by DSC measurements, in situ powder and single-crystal X-ray diffraction as well as Raman spectroscopy. Cooling/heating experiments using the different techniques prove that alunogen undergoes a reversible, sluggish phase transition somewhere between −30 and −50 °C from the triclinic room-temperature (RT) form to a previously unknown LT-polymorph. A significant hysteresis for the transition was observed with all three methods and the transition temperatures were found to depend on the employed cooling/heating rates. The crystal structure of the LT-modification has been studied at −100 °C using single crystals, which have been grown from an aqueous solution. Basic crystallographic data are as follows: monoclinic symmetry, space group type P21, a = 7.4125(3), b = 26.8337(16), c = 6.0775(3) Å, β = 97.312(4)°, V = 1199.01(10) Å3, and Z = 2. Structure analysis revealed that LT-alunogen corresponds to a non-stoichiometric hydrate with 16.61 water moieties pfu. Notably, the first-order transition results in a single-crystal-to-single-crystal transformation. In the asymmetric unit there are 2 Al-atoms, 3 [SO4]-tetrahedra, and 17 crystallographically independent sites for water molecules, whose hydrogen positions could be all located by difference-Fourier calculations. According to site-population refinements only one water position (Ow5) shows a partial occupancy. A comfortable way to rationalize the crystal structure of the LT-modification of alunogen is based on a subdivision of the whole structure into two different slabs parallel to (010). The first type of slab (type A) is about 9 Å thick and located at y ≈ 0 and y ≈ ½, respectively. It contains the Al(H2O)6-octahedra as well as the sulfate groups centered by S1 and S2. Type B at y ≈ ¼ and y ≈ ¾ comprises the remaining tetrahedra about S3 and a total of five additional “zeolitic” water sites (Ow1–Ow5), which are not a part of a coordination polyhedron. Within slab-type A alternating chains of (unconnected) octahedra and tetrahedra can be identified, which are running parallel to . In addition to electrostatic interactions between the Al(H2O)63+- and the (SO4)2−-units, hydrogen bonds are also essential for the stability of these slabs. A detailed comparison between both modifications including a derivation from a hypothetical aristotype based on group-theoretical concepts is presented. Since alunogen has been postulated to occur in martian soils the new findings may help in the identification of the LT-form by X-ray diffraction using the Curiosity Rover’s ChemMin instrument or by Raman spectroscopy.