In this study the phylogenetic relationship of the Jajrud River Brown trout Salmo trutta L. 1758 in the Namak Lake basin was analyzed using complete D-Loop sequence of the mitochondrial genome, in comparison with other Iranian populations of the species. According to the results, Jajrud brown trout, like other Iranian brown trout populations, belongs to the Danube phylogenetic lineage. The observed haplotype in the Jajrud population is similar to the reported haplotype in the Karaj River. The results showed that the Namak Lake basin brown trout affined to the Caspian populations, while its relationship to Salmo macrostigma was not confirmed. Although the Karaj and Jajrud River haplotypes have not been observed in the Caspian Sea basin, because of its relationship to other Iranian and Danube haplotypes, it is plausible to consider this haplotype as an old haplotype that also may exist in the Caspian Sea basin. According to the biology of brown trout, geographic characteristics, and based on the properties of the Namak haplotype, it may be inferred that the Namak basin brown trout populations are native to the habitat and not introduced.