The synthesis and crystal structure (100 K) of the title compound, ammonium bis[salicylaldehyde thiosemicarbazonato(2-)-κ3O,N1,S]iron(III), NH4[Fe(C8H7N3OS)2], is reported. The asymmetric unit consists of an octahedral [FeIII(thsa)2]- fragment, where thsa2- is salicylaldehyde thiosemicarbazonate(2-), and an NH4+ cation. Each thsa2- ligand binds via the thiolate S, the imine N and the phenolate O donor atoms, resulting in an FeIIIS2N2O2 chromophore. The ligands are orientated in two perpendicular planes, with the O and S atoms in cis and the N atoms in trans positions. The FeIII ion is in the low-spin state at 100 K. The crystal structure belongs to a category I order-disorder (OD) family. It is a polytype of a maximum degree of order (MDO). Fragments of the second MDO polytype lead to systematic twinning by pseudomerohedry.