Three moderately acidophilic, halophilic archaeal strains, MH1-243-3T, MH1-243-5 and MH1-243-6, were isolated from a commercial salt sample made from seawater in Okinawa, Japan. Cells of the three strains were pleomorphic and stained Gram-negative. Colonies of the strains were orange-red-pigmented. Strain MH1-243-3T was able to grow at 15-27 % (w/v) NaCl (optimum 24 °C), at pH 4.5-6.5 (pH 5.5) and at 35-50 °C (45 °C). Strains MH1-243-5 and MH1-243-6 grew within slightly different ranges (shown in text). The 16S rRNA gene sequences of the three strains were identical, and the closest phylogenetic relative was Halarchaeum salinum MH1-34-1T with 97.0 % similarity. The rpoB' gene sequences of the three strains were also identical, and the closest phylogenetic relative was Halarchaeum acidiphilum JCM 16109T with 92.0 % similarity. The DNA G+C content of MH1-243-3T, MH1-243-5 and MH1-243-6 was 65.2 mol%. The levels of DNA-DNA relatedness amongst the three strains were 84.1-99.8 %, while that between MH1-243-3T and H. salinum MH1-34-1T was 30.6 % and 31.6 % (reciprocally), and those between MH1-243-3T and type strains of other species in the genus Halarchaeum were 42.3-29.4 %. Based on the phenotypic, genotypic and phylogenetic analyses, it is proposed that the isolates should represent a novel species of the genus Halarchaeum, for which the name Halarchaeum grantii sp. nov. is proposed. The type strain is MH1-243-3T ( = JCM 19585T = KCTC 4142T), isolated from commercial sea salt produced in Okinawa, Japan. MH1-243-5 ( = JCM 19586) and MH1-243-6 ( = JCM 18422) are additional strains of the species.