Wiedemann-Steiner syndrome (WDSTS) is an exceptionally rare genetic syndrome characterized by postnatal growth retardation, facial dysmorphism, hairy elbow, and short stature. It is known that occurrence of WDSTS is due to mutations in KMT2A gene. It is noteworthy that, not a great number of WDSTS have been identified yet, thereby new phenotypes and features continue to be added. In this report, we describe a 5 year old male patient presented with developmental delay, hypothyroidism, facial dysmorphism and behavioral signs such as autistic spectrum features. By Whole Exome Sequencing (WES), a new mutation in KMT2A was found and WDSTS was diagnosed genetically. According to genetic test, a variant in exon 27 of the KMT2A gene c.6647delT (p.Pro2215fs) was found. This mutation was not reported previously, also this case was the first WDSTS diagnosed in Iran. This syndrome is a rare genetic disorder representing a broad range of phenotypes. The mentioned low frequency emphasizes the importance of a phenotype-genotype correlation to be established. The phenotype comparison between our case and previously reported patient did not reveal any difference related to age or sex in patients with WDSTS.