Paralactaga williamsi, a five-toed jerboa, is known to occur in Anatolian Turkey, northern Lebanon, Armenia, Azerbaijan and western Iran. Here, we report the first record of this species in Kopet-Dag Mountains, northeastern Iran. Species identification was based on external morphology, skull and molar teeth morphology and morphometrics studies, as well as molecular analyses. Brief notes on the ecology of the species are also provided. This new record expands the distribution of P. williamsi in Iran about 850 km eastward, however further sampling will be needed for a better judgment on the taxonomic status of this species in eastern Iran and to determine the patterns of its distribution. Since the specimen did not group with other P. williamsi in the cytochrome b analysis, we provisionally classify the specimens as P. cf. williamsi. [ABSTRACT FROM AUTHOR]