A mesophilic, neutrophilic and aerobic, ammonia-oxidizing archaeon, strain EN76^T, was isolated from garden soil in Vienna (Austria). Cells were irregular cocci with a diameter of 0.6–0.9 µm and possessed archaella and archaeal pili as cell appendages. Electron microscopy also indicated clearly discernible areas of high and low electron density, as well as tubule-like structures. Strain EN76^T had an S-layer with p3 symmetry, so far only reported for members of the Sulfolobales . Crenarchaeol was the major core lipid. The organism gained energy by oxidizing ammonia to nitrite aerobically, thereby fixing CO₂, but growth depended on the addition of small amounts of organic acids. The optimal growth temperature was 42 °C and the optimal pH was 7.5, with ammonium and pyruvate concentrations of 2.6 and 1 mM, respectively. The genome of strain EN76^T had a DNA G+C content of 52.7 mol%. Phylogenetic analyses of 16S rRNA genes showed that strain EN76^T is affiliated with the recently proposed phylum Thaumarchaeota , sharing 85 % 16S rRNA gene sequence identity with the closest cultivated relative ‘Candidatus Nitrosopumilus maritimus’ SCM1, a marine ammonia-oxidizing archaeon, and a maximum of 81 % 16S rRNA gene sequence identity with members of the phyla Crenarchaeota and Euryarchaeota and any of the other recently proposed phyla (e.g. ‘Korarchaeota’ and ‘Aigarchaeota’). We propose the name Nitrososphaera viennensis gen. nov., sp. nov. to accommodate strain EN76^T. The type strain of Nitrososphaera viennensis is strain EN76^T ( = DSM 26422^T = JMC 19564^T). Additionally, we propose the family Nitrososphaeraceae fam. nov., the order Nitrososphaerales ord. nov. and the class Nitrososphaeria classis nov.