Strain YSM-79T was isolated from commercial salt made from seawater in Yonaguni island, Okinawa, Japan. The strain is an aerobic, Gram-negative, chemo-organotrophic and extremely halophilic archaeon. Cells are short rods that lyse in distilled water. Growth occurs at 1.5–5.3 M NaCl (optimum 2.5–3.0 M), pH 5.0–8.8 (optimum pH 5.2–6.3) and 20–55 °C (optimum 40 °C). Mg2+ is required for growth, with maximum growth at 200–300 mM Mg2+. Polar lipid analysis revealed the presence of phosphatidylglycerol, phosphatidylglycerophosphate methyl ester, sulfated diglycosyl diether-1 and five unidentified glycolipids. The G+C content of the DNA was 64 mol%. On the basis of 16S rRNA gene sequence analysis, strain YSM-79T was determined to be a member of the family Halobacteriaceae, with the closest related genus being Halobacterium (94 % sequence identity). In addition, the rpoB′ gene sequence of strain YSM-79T had <88 % sequence similarity to those of other members of the family Halobacteriaceae. The results of phenotypic, chemotaxonomic and phylogenetic analysis suggested that strain YSM-79T should be placed in a new genus, Salarchaeum gen. nov., as a representative of Salarchaeum japonicum sp. nov. The type strain is YSM-79T ( = JCM 16327T = CECT 7563T).